Hi there,
JohnNY123, sorry for not reacting before, I was just doing some testing here and wanted to wait a bit before coming with a conclusion.
First of all, I've been using the SmartSleep app, I don't fancy having to wait for the MacBook to write the hibernation file first before sleeping, but I understand its usefulness when battery is low.
Initially, I thought SmartSleep could be causing the issue but found it hard to believe since all it does is monitor the battery level and switch the sleep mode accordingly. Then, as in the original post, I realized this issue of switching between sleep and hibernation during sleep arised *only* after the last supplemental update. It actually can be useful if your laptop has an SSD and concerve battery life during sleep, but with an HDD it's not as nice when it comes to waking up. Maybe it's related to the PowerNap feature?
Anyway, following the tip of @egsl, I decided to investigate the autopoweroff option. And I came across this: http://apple.stackexchange.com/questions/72889/mountain-lion-aggressively-hibern ates
I suppose that "pmset -g" lists all options that can be tweaked via pmset. So I went on and did:
sudo pmset -a autopoweroff 0
Voilá, problem solved. Like someone in that stackexchange thread, I find it very unusual for this kind of behavior/option to start happening after an OS minor update. Surely it is a bug for 2012 conventional MacBooks. My 2010 MacBook Pro doesn't behave like so. Indeed, even after the last Mountain Lion update,, "pmset -g" won't even list an option for autopoweroff. On my 2012 MacBook, there's now also a "womp" option, short for "Wake up on magical ethernet package" = PowerNap. My original hypothesis seems to be further correct in that this update "considers" all 2012 latops as MacBook Airs or retina MacBooks, all systems with an SSD, thus there's a bug.
Ok, to summarize, sudo pmset -a autopoweroff 0 seems to fix the issue. Sorry for the long post 🙂
Thanks!