If you are (or have been) in the habit of sticking pre-numbered labels in books and then releasing those books without entering the book details yourself then this is actually "expected behaviour". These BCIDs are only assigned to a certain book as soon as someone - you or a finder - actually enters the book details (until then the database has no way of knowing that such a BCID is actually used).
If on the other hand you always enter the book details yourself, then all these prenums are unused yet. In the database I see quite a few unassigned batches of 10 and two completely unassigned batches of 100 (all BCIDs of one batch have the same prefix). In your list of prenums you should see a link "reprint this set of labels" just below every entry. This lets you download this batch of labels again. BEWARE: If you do this for a set of which you already have used some of the BCIDs you'll have to take care yourself to weed those out.
In the database I see quite a few unassigned batches of 10 and two completely unassigned batches of 100 (all BCIDs of one batch have the same prefix).
Over the years, I found a number of batches in my unused PNL list which I certainly never had seen before. For example, I am rather sure that in earlier years it happened several times that I wanted to download one batch but the website assigned two while only showing one of them to me. One idea is that I might have clicked twice before the download of the first file actually started, thus canceling one download. However, I don't know what really happened.
A few months ago, I once got a 20-batch (in a two page PDF) which I certainly hadn't ordered as a 20-batch, simply because you can't do that. And I used up all 20 labels successfully. Luckily, I realized that something was weird and the file had two pages. Since I usually copy the numbers into other label designs, I don't print the original file and since I never use 100-batches, my files should all have only one page...
As to where those might come from: it's possible to accidentally click on the "request 10/request 100" link on the prenumbered-BCID pages, or on the "labels" page, though I'd imagine it's less likely to have done so more than once! Or perhaps ekorren's suggestion about the accidental double-click is involved - if the site's slow or one's computer is being iffy, it's certainly possible to assume nothing's happened yet and click again; I think I wound up with some bonus sets of prenums due to that once or twice.
Another possibility - have you ordered any prenumbered labels from the supply store? If so, those prenum-BCIDs will appear on the same list as those you allocated via the "request" or labels-page "download" features.
You might be able to work out when those sets appeared by checking the public portion of the BCIDs against your most recently registered books, though I don't know if that'll help figure out what happened.
Not unless you have already used the number, but not activated it. If you released a book and didn't activate it, it will still show on that list as not used, so if you re-use it two books will have the same number.
Seeing this posting it reminded me I had 3 or four to use up, so having bought too many books today, I sat down to register them and found I have 2 pages of pre-numbered labels. I certainly haven't ordered any since before the convention in Melbourne so I don't know how they got there. I must have the same hiccup as you.
The number-range can give you some idea of *when* they appeared, if not how... Though I do recall that pre-site-rebuild, there were times when an attempt to download prenumbered labels from the "labels" page would (apparently) fail, causing me to try again - and leaving an extra set of BCIDs allocated to me, BCIDs that were invisible before we got that "show me all my unused prenums" feature. So if those BCIDs are from several years ago that could be their origin.
And a reminder (apologies to everyone who knows this already!) that any sets of prenumbered labels bought from the supply store or downloaded from the book-labels page will put those BCIDs onto your profile-page list, where they'll stay until activated. So before using up numbers from that list, make sure you don't already have printed or supply-store labels on hand (or in not-yet-activated books), or you could wind up with multiple books with the same BCID. FWIW, I came very close to doing this myself; I had one very old (as in "six-digit-number" old) BCID on my unused-prenums list for the longest time, and could not figure out why I hadn't used that label. I was on the point of deciding I'd lost it and using the number anyway, when - going through one of several boxes-of-stuff in the basement - what should turn up but that one lone prenumbered label!