Statesman, chances are that in a group of more than 4 people, there will be someone that cannot make it into a danger zone at higher lvls. this means that for a team of friends or SG-mates someone usually has to drop from the team in order for the rest of the group to get decent XP. Otherwise, based on the numbers you presented earlier the team will not earn XP as fast.
This kind of forces people to deal with slower xp rates just so they can play with friends.