As brake bads wear, the caliper pistons travel farther out causing the fluid level in the resivor to go down. People see this and top it off, which is ok, but when you go to put brake pads in, and you contract the piston, the brake fluid in the resivore rises.
I usually just take the cap off befor I start and look to see how full it is, remove some if necessicary. However, the best thing you can do is crack open the bleeders and bleed excess fluid out that way. However in MN, after about a year or so in the salty elements, trying to crack open a bleeder could result in replacement of a caliper.