It can be a combination of several things but my 1st is the tire psi- drop it down and second, the brunishing of the brakes...whether they are high or low end linings, given the variable damage (wear) to the rotors, the bedding of the pads will vary based upon type and amount of surface contact- which changes the amount of "bite', so i would say that what you are experiencing is not unexpected. Once the rotors are turned/replaced and new pads installed everything should be fine.