Group Rotation Clock and counter Clockwise

[Zendrael]

Hi!

I'm rotating a group clockwise and counter clockwise with a button that does

mygroup.rotation += 90; //for clockwise

mygroup.rotation -= 90; //for counter clockwise

but the group, rotated from its center, does not rotate 90 degrees... why?

It rotates sometimes more or less, but never 90 degrees...

Any ideas? I'm not using physics.

Thanks