3 month epc = average earnings per click over last 3 months
7 month epc = average earnings per click over last 7 months
If 3 month epc is higher, then you know the offer has been doing better overall in the last 3 months than the last 7 months, and vice versa.
Network EPC is based on an average of all the affiliates' epc's, so all because it says $1 epc doesn't mean you will make the same, you might do a lot better or a lot worse.