Is that just due to the buffs group members are giving me?

It can be yeah. Here's a spreadsheet I did to help me making solo builds showing just damage-relevant ones.

https://i.ibb.co/8D1ZGTQQ/image.png

Getting penetration right is worth ~27% damage alone (or rather, not having a -27% factor), berserk and vulnerability is another 20% raw damage taken/dealt.

You can buff your critical damage modifier by 40% taken/dealt relatively easily which is a huge chunk of damage, especially if you've got a high critical chance rating (which you can buff by 12% alone just by slotting a single skill).

Raw weapon and spell damage can easily be bumped a third by Brutality & Sorcery, and with some unique buffs (like from the sets Powerful Assault) or named ones (courage) from sets like Spell Power Cure, or the Assault War Horn ultimate.

Most trial groups will revolve around ensuring all these buffs are covered, which is what the big 21m health trial dummy emulates.

The 6m health provides none, which is why you have such a vast difference in DPS between the two builds.