let's exaggerate the problem:

Month 1: 1 person, 0 redhead (0%)
Month 2: 99 people, 99 redhead (100%)

method 1:
(0% + 100%)/2 = 50%

method 2:
(0+99)1+99) = 99%

the first method gives an equal weighting to each month, regardless of the number of people in it's measurement.
the second number gives equal weighting to each person, regardless of the month he was observed in.

now, as Tom said, it really depends on what exactly is the question you are trying to answer.