The +/- signs after each character represents the number of times more or less that the character was seen this month than the previous month.
For example, Michael Baldwin- 17 episodes (-3), means that Michael was on 17 times in November and the (-3) means he was on 3 times less in November than he was in October which would have been 20 episodes. So for another example lets say at the end of December Michael Baldwin is on 18 times, his episode count for December would read Michael Baldwin- 18 episodes (+1), the (+1) meaning he was on one more time in December than he was in November.
I hope that makes some kind of sense! LOL!