...what did you expect? 4 of them have mr and only 2 of you have armor... It seems pretty normal to me.
He also happens to get damage from MS, which Diana doesn't, and Ghost + Boots + Trinity force gives him a pretty decent chunk of AD. And if he was running predator, which Hecarim usually does, that's a massive chunk of damage because of his MS scaling.
He also had a few more assists than you did.
Plus, as mentioned above: MS scalings on Hec plus Pred keystone
He's also 2 levels up on pretty much everyone including you.
It's almost as if he is a different champion with different scalings or something.
Not to mention, Hec is up 2 levels, so of course he is going to do damage.