iMac late 2012 strange powersave behavior

so folks...i have set it for:

turn off display after 15min
sleep computer after 30min

what happens is that the screen dims after couple of minutes and display turns off way before 15min.

also sleep function is strange...it comes back to life almost instantaneously, while other times i get that "faded out" background and the horizontal progress bar.....

so questions:
- why does the screen sleep always faster than what's set in power preferences?
- which one is the real sleep mode?

thx!