Well they are only supposed to be hand tight, to stop the disc moving when the wheel is off. Some people seem to think that bolt holds the disc to the hub on it's own and crank it right up (not realising the wheel and wheel bolts are the main parts that hold the disc in place). Hence overtightened small screw will be seized, and round when you try remove it.
If it doesn't go easy, then use an impact driver or you'll be drilling it out.