I can't tell you for sure. They do seem somewhat high, I admit.
But he often has his Sprites helping him. Each additional person (or sprite in this case) doing a teamwork test raises your Limit by 1, in addition to adding it's successes to your dicepool. It's possible that he and the GM just agreed "Hey, unless I say anything different, just assume I have two of my sprites assist me on every roll, they're just buying hits, so with a Level 6 I get +1 limit and +2 dice"
We also don't really know how much Karma they have at this point, he might have raised most his mental stats up to his natural maximum, being 6 as a human. I don't think Submersions increase the max for those, just for his Resonance, but having it at 6 is not bad. Of course through Submersions he could have picked up some of those Echos that allow him to boost some matrix stats without raising the Mental stats. Might have some of those, I guess.
There's Bioware that increases some of those attributes. During that 1 year downtime, they were apparently allowed to make changes and retcon their characters. Possible he retconned some of that into him, maybe even through Prototype Transhuman. Don't know that though, just a guess.