The S# was at about #1000 when signatures were announced as "sold out" beginning of 2012.
So it seems that the S# numbers contain holes for the SSL# numbers, as we think they have a total of 1000 and the highest S# at that time was #1000.
Meanwhile the S# is at 1278, so there must have been at least 278 cancellation, deferrals or processing glitches. Theoretically possible that in some cases, things got messed up during the reservation process and they created a new reservation instead of fixing the existing one, even though nobody cancelled.
In any case, if we assume that there are still about 1000 signature cars being delivered, then the numbering through 1278 (now) contains 278 holes.
So the number of still valid non-SSL signature reservations is now maybe 500 less than 1278, but 244 of those 500 are still valid SSL reservations.
The sequence of numbers up to 1278 includes plain valid S# (non-SSL) reservations, gaps for SSL numbers (where the SSL number is not the same number as the number of the gap), and gaps for cancellations/glitches.
According to this interpretation, assuming a total of 1000 signature cars will be built, there are 278 cancellations/glitches, plus some cancellations before the last compression(s) of the list(s) took place. (Except for changes in the last few days).