from posts I have seen on r/diablo, people are guessing that before the patch ilvl 63s dropped at an approximate rate of 20% in act 3 (honestly, I have an issue with how the guy has done the numbers, so it might be lower).
now they are dropping at 8% in Act 3 (confirmed by numbers and stated by blizzard), which is obviously a significant drop.
but it's fair enough I guess. overall it's easier to farm there, as well as probably wanting to redistribute the drop rates instead of add more. it's a way of controlling the amount of ilvl 63 items in the market.
aetherfox wrote:BronzeD: Since Act 3 and Act 4 have the same loot distribution table, is there any reason to do Act 4?
do what you find easier. pre patch people used to smash siegebreaker runs, but now he has reflect damage it's not as easy. the change to stacked drop rates should also play into it. at 5 stacks each elite mob is guaranteed to drop 1 rare (up from 0), and bosses are guaranteed to drop 1 rare (down from 2). so doing a boss shouldn't be a target anymore, just a nice bonus at the end as they are typically fairly easy.
for anyone that isn't up with everything and to put everything in one place, the drop levels for ilvl 63 items are now:
act 1: 2% (from 0%)
act 2: 4% (from 0%)
act 3 and 4 (as well as whimseyshire): 8% (from ~20%).
below ilvl 63 there have been other changes (increase for hell act 4, act 1 and act 2 I believe), but not many people about that.