+/- is not something to use for an argument because it doesn't take into account the situation of the team. It would work if all things were created equal, but they're not.
Draisaitl and McDavid are the backbone of the offense, so they shoulder the most of the offensive load (and many nights, all of it). If they don't produce, the Oilers get annihilated. So if you play high minutes trying to produce offense as the only viable source, and if that viable source doesn't produce, of course your +/- is going to tank.
Matthews is in a completely different situation. He has a grand canyon of offensive talent around him. That takes off a lot of the pressure off of his shoulders in order to produce offensive. It's easy to play with great players. Everyone is capable of scoring, and the leafs are a high octane offense. That's why his +/- is so great.
How do people not understand this
Imagine Matthews as the sole means of offense on Oilers and put McDavid and Drai on the Leafs. No Matthews fan would touch +/- with a ten-foot pole.
Case closed.