First, I don't believe "average group level" is a thing for XP calculations, insofar as, lower level players have their potential XP capped, potentially leaving more XP on the table to be divided up to higher level players. The big thing is the group XP bonus. Think of it this way. At level 30 or whatever, the maximum XP per kill you can get from a non elite equal con mob is 195 with an additional 5% per mob level higher than your level, up to 4 levels. Elites 2x. This is when you're solo. XP gets divided by the group, but then modified by a group bonus modifier.
When you're extremely under-leveled in a dungeon, you're butting up against this cap. At 35 in ST, your XP cap is something like 500 (+4 level mobs) XP from a solo kill. Of course this has to be split between the group, while a level 48ish elite mob is worth 570ish xp base solo. So, at level 35 in ST you said you were getting 60ish xp per kill I think, but at level 48, you'd be getting almost certainly around 100 xp per kill.
Basically, your group is killing mobs slower, and you're still getting level 35 XP rates, whereas, if you were in say Cath, you'd be killing level 38-40 mobs, they will die much faster, and you're getting essentially the same XP per kill as you would in ST.
Group XP splits, are definitely relative to the overall XP cap at any given level. So if a mob is worth 600 XP say, and the group is all level 25's and one 60, the cap is probably somewhere around 1/5th of what their solo base cap is, so 34 (add the group modifier which I think is 40% for 5 players), so 48 xp per person, but that is only 34*4 or 136 xp out of that 600 XP that those 4 level 25's are actually sucking up. Which means that a huge chunk of that XP is actually being left on the table.
This is why ST is hideously slow for a level 35. You're being boosted on relatively high level mobs. Your character is not capable of actually soaking up the xp that is being generated per kill, so the rate of killing is low, and the xp per kill is low. Go to SM with a single level 60 chain killing, and the rate of killing goes up, xp per kill stays roughly the same until you start to con blue to the mobs.
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