The thing is that medals have levels to which you can upgrade them. For example, to get the first level of the "Veteran X" medal, you need 100 kills. So if you have 590 kills in the "Heroes Killed" field, but the medal's progress bar says 490 kills, that means 100 kills went towards the first level of the medal, and the other 490 kills go towards the second level of your medal. The overall number of kills made by a certain hero is still correct, and the medal's progress is identical to your statistics. 

It also works the same way for the medal that counts how many times you've revived your allies.