So the x5 of the players killed were of no threat to your team? Those x5 players killed were all peasants, and nobodies? Killing is still killing, regardless if your class makes it easier or harder to do. That's what wins rounds, you don't win rounds unless enemies die. Assists are just as important (so don't get me wrong about that) but ultimately it still leads to someone dying.
I don't know why people say "cav is easy mode, so their contribution to the team isn't that important". Using rocks and being decent isn't as valuable as using an actual weapon and killing mass numbers of people.
There's a reason cav tend to dominate scoreboards. Whenever I spec someone who plays cav it's 20% them making good plays and 80% people literally throwing themselves on top of a waiting heavy lance.
I'm an awful cav player, I never played cav in native and never played it for more than half a gen in the past in cRPG. I decided to go cav for a gen about a month or so ago, and ended the gen with a 2.5:1 KD. Now, if I can get that ratio as godawful cav, I can only imagine what people who know what they're doing can rack up.
I want to stress that cav getting monster KD doesn't make them OP. On the contrary, in 1v1 situations I rarely lose to cavalry. However there are so many people that don't know what they are doing against cav that you have to really screw it up if you get a poor score consistently.