Ovechkin and Backstrom had a decent gap for a while. Over 2M. Now Carlson splits the difference.

While it's an interesting hypothesis, and there are some teams that have refused to sign new guys for more than one current star already earned... a few years of arbitrations have standardized a lot of the negotiation process, by listing out what sorts of stats are worth how much to other players the same age and with similar contract status. It's not really about what Matthews makes. It's about what other guys the same age, position, and productivity are making.

That seems to be the general consensus. The reality is that the Leafs have 3 players with very similar production over the last 2 years and yet somehow in every Leaf thread there are millions separating these 3 players.
A couple teams have big salary gaps between their stars, the Caps as you mentioned, but most have a salary structure and the more good players they have on their rosters, the less the stars make as a result.