And when they don't fit because they're too thick, when you take them back to the store bring a pair of calipers (the dial, or digital type, not the brake type!) with you.
Luckily you have a better chance of getting a correctly sized pair, but it's still hit or miss. Lots of reports of replacement pads for both GM and Ford applications being too thick to fit back over the rotor.
The cure is to either swap them for a good pair (hence the measuring calipers), or sand them down on a flat surface to fit. Usually it's only a few thousandths, but that's more than enough to keep them from fitting the first time.
Paul