It is the very problem of the 'cumulative' update regime. I have read the excuse mentioning something about an error in the update infrastructure, but frankly I don't buy into it as it seems more of a tactic to avoid criticism for the cumulative updates they introduced last autumn.
The effect of this is that we're now left open and vulnerable to several security holes instead of just one which would have been the case with the older regime of separated updates.
It really was the glory of service packs when they worked as they saved some hours of patching hell (mind my language, but anyone who's been in this for some years will agree that it is indeed the correct wording)
In my view, 'Patch Tuesday' should be individual updates and then after the initial release, they should be put into cumulative updates come next patch Tuesday. This way we would be able to avoid 'patching hell' and receive the best of both worlds. Of course, that is from a consumer standpoint only, but it would avoid the situation we're in now