Change in sleep behaviour on MacBook Pro and ignoring settings on Yosemite
I can't tell why you Mac wakes up, but here is how you could find out:

Type pmset -b -g assertions in your terminal. That will list the drivers that have requested power setting overrides when running on battery.

You can also inspect the power management log, using pmset -g log.

For example, on my 10.10.5 MacBook Pro 2015, I found out that leaving my Sandisk Ultra fit USB3 memory stick plugged in makes my Mac wake up every 3 hours. Here's why:

$ pmset -b -g assertions
2015-10-19 18:38:32 +0200 
Assertion status system-wide:
   BackgroundTask                 0
   ApplePushServiceTask           0
   UserIsActive                   0
   PreventUserIdleDisplaySleep    0
   PreventSystemSleep             0
   ExternalMedia                  1
   PreventUserIdleSystemSleep     0
   NetworkClientActive            0
Listed by owning process:
   pid 51(powerd): [0x000000040008012e] 03:37:52 ExternalMedia named: "com.apple.powermanagement.externalmediamounted" 


ExternalMedia is set to 1 by process powerd. 

Unfortunately I haven't found out yet how to prevent this. I looked for a "com.apple.powermanagement.externalmediamounted" plist file but could not find any... So I don't know if these driver overrides can be changed or disabled. :(  Anyway, I expect it is driver dependent, so changing the behavior should also be driver dependent...

This detailed web page seems to indicate that, for external media altering sleep behavior, the only option is to unmount or eject the external media. Based on the driver the external media uses, eject is sometime required while for other unmount is enought.