Sleeping mode and start up inconsistens behaviour
You are experiencing more or less the same as me. However I can't change my settings right now. 

Normally your mac will simply sleep, i.e. turn everything into a low power mode and supply the RAM with electricity so it does not get deleted and you can quickly restart working. Then after one hour or so autopoweroff will kick in to save more energy (in order to comply with EU regulations). This is normally set by the autopoweroffdelay. As far as I know this is more or less a suspend-to-disk mode. That means the contents of your memory (RAM) is written to your disk and the RAM is taken offline. So when you next start your Mac, it will first have to load the RAM back from the disk which can take 10 to 60 seconds depending on your drive's speed and RAM size and usage.

To make things more complicated there is also a standby mode.

Now the implementation with FileVault2 is IMHO a bit unfortunate. What happens is that your Mac will first need your password to decrypt your disk. Only then it can start the RAM restoration procedure. This is particularly annoying if the acwake feature is on. In this case the Mac will power on, if you remove the power cord. When I first bought my Mac, it happened to me a couple of times, that I had to leave in a rush, removed the power cord, put the Mac into my bag and when I opened it the battery was half drained. The problem is that it will not sleep again once it is waiting for your FileVault2 password. 

Therefore I disabled acwake with the following command: sudo pmset acwake 0.

In order to secure your FileVault2 further, you can use the command sudo pmset -a destroyfvkeyonstandby 1 hibernatemode 25. With this setting the FileVault key will be destroyed once the Mac goes into standby. That means that only suspend-to-disk is used. You may also want to reduce the standbydelay.

You can list your timers and options by typing pmset -g into the terminal. You will find more info by typing man pmset.