T/O EPR settings are a static value. That's why they must be set before Ram effect occurs. If you keep a close eye on the EPRs on your takeoffs, you will notice that around the time you're raising the gear, they will be approx. .04 lower that they were at the start of the roll. (This will vary, of course, with a/c type and ambient conditions). If you push them up, you will be overboosting.
GA EPR is designed to take Ram effect into account. You will notice (if you compare TO and GA EPR settings) that GA will be lower, even though both will produce the same thrust.
Mutt gave you some good examples of differing thrust with differing conditions.