Just about everything really.
Hedman is a great dman, no doubt. But this year, he's being deployed as mostly an offensive dman. He gets the most offensive zone starts on the team, while McDonagh and Stralman take all the defensive zone starts. McDonagh and Stralman are Tampa's go to against other teams top lines as well. So, while Hedman has managed a pretty solid 53.6 CF%, and has some decent, but not spectacular advanced stats, he's doing that based on some pretty sheltered ice time. Yep, he's actually sheltered this year, and all the stats back that up.
Babcock meanwhile spreads his ice time out, and Rielly is middle of the pack on the Leafs, and is frequently the go to guy for Babcock against the top opposition (although Gardiner has been getting a lot of that this year).
Defensively, Hedman is faring a little bit better this year, but not huge margins. Rielly is holding his own, Hedman is a little above average, but given the easy assignments.
Offensively, again, because the ice is spread out in Toronto, Rielly doesn't get the PP time Hedman does either. Hedman averages 3:21 per game right now, Rielly just 2:38. And yet, is still smashing him in production. And you can't really use the "it's his linemates" excuse when comparing these two either.
So, when you're looking at two dmen, one getting favourable ice time, and making out marginally better, defensively, but getting completely destroyed offensively, by a wide margin, yeah, it's pretty easy to say Rielly is having a better season than Hedman.
But yeah, Hedman is money in the bank. He's awesome. He's just not having the season Rielly is.