This means Marcus is getting his share of the kills in all of those levels that he is a complete experience drain.
So what if he's an Exp drain? Matters for the Exp ranking, but I doubt you care about that. In all other senses he is no more an Exp drain than, say, Lowen. Either way, if I give them 10% of available kills, that's 10% that nobody else in my party can get. Giving Marcus 70 kills = giving Lowen 70 kills. In comparing Marcus and Lowen, who are available for exactly the same number of maps, it is only fair to give them the same kills. Yes, Marcus getting those kills means nobody else can... the exact same way Lowen prevents other people from getting those kills. The only person who suffers for Marcus getting less Exp than Lowen is Marcus himself... and this is factored into his calculated Exp gain.
(Also, Tal is pretty much correct.)