Does it have to do with the troops used? Lower age troops give lower points perhaps?
Looking at this again, it is an interesting question. I had never considered this in regards to GE Expedition Points, and I'll say up front that I don't have the definitive answer. Here is my thought on it: It is possible that it could have a small impact if you use lower age troops, but only if you sustain damage yourself in fighting a GE encounter.
The reason I say this is because of how battle points are determined. When figuring battle points, you get points based on the value of the units you kill, minus any damage you take. Higher age units generally have higher values than lower age units. If you take no damage in a battle, then the age of your units makes no difference to the battle points earned. However, if you take any damage then there would be fewer points deducted if you are using lower age units.
An example:
You attack with 8 Mounted Archers (EMA) worth 280 points each and kill 8 Armored Infantry (EMA) worth 230 each. If you do not take damage, you will receive 1,840* battle points. 230x8=1840. If you lose one Mounted Archer, however, you would receive 1,560 points. 1840-280=1560.
If you were to attack with 8 Archers (Iron Age) instead, they are worth 180 points each. You would still receive 1840 battle points if taking no damage, but if you lose one Archer you would receive 1,660 points. 1840-180=1660.
Does this difference in battle points translate to GE Expedition Points? That I do not know. Maybe, maybe not. Anyone care to test? (It would take two GE rounds, as you would have to fight the same exact encounter twice. You would also have to sustain the same amount of damage both times, once using lower age troops and once using current age troops. And you would have to win the encounter both times.)
*For purposes of simplicity I am ignoring the boost to battle points that you get from having an Enthusiastic population.