I think it is just weird, that the update for older OS versions is so much smaller (71 MB) than the update for the newest version (372 MB).
And on the Apple website it says you can use the 71 MB update also for 10.5.5.
This is just totally weird.
Almost looks like, they replaced something in 10.5.5 which they are now again replacing with something that was already there since the beginning.
But still does not explain why the 71 MB update is also supposed to work with ...