Because Mackinnon signed in the summer of 2016 after 3 years in the league with 24G/63PTS, 14G/38PTS and 21G/52PTS. Funny enough he also only payed 1 full season out of his first 3 but that's besides the point. It was a gamble based on how high he was touted coming into the league and the downward trend, he absolutely became a steal of a contract, same as Schiefele who signed that summer as well.
So, we've covered that Mackinnon started off slower than Matthews and got a contract that 2 years after he signed it, became a bargain. How many people would have predicted that? Now people are also predicting that Matthews cannot already live up to his contract because of what exactly? Starting off his career with stronger numbers than Mackinnon? I'd love to be able to hear the argument as to why Matthews has failed his new contract already.
Because that's when Auston Matthews came into the league and when he started to earn NHL points? That's my point, Mackinnon came into the league and started off slow and is now producing at what people were predicting he would. Matthews came in with a stronger first 3 years performance and based on how players (at least most 1OA picks) progress, it will continue to be upwards. How much and for how long, I'm not sure. But to be in the NHL for only 3 years now and be top 5 league wide in goal scoring by almost every metric, is astounding. Imagine improving on that? Like I said, I doubt he gets the high point totals and may have a couple 100pts seasons here and there but I expect 45-50G, mostly EV, and 40-45A. The premium being goal scoring at even strength, if you don't value that, cool I guess.