You're assuming that no one had Kucherov that high before this playoffs, that his ranking is solely because of the playoffs and that if his ranking is because of the playoffs, it means he has to be above everyone he's had a better playoffs than. These are all flawed assumptions.
Kucherov was widely viewed as 2nd to McDavid after his 18-19 season, and while some had him fall lower after 19-20, his 30+ point playoffs last year still had him near the very best. If he wasn't considered in that group of top 3/4 before this playoffs, it was either because people were worried about how he'd bounce back from surgery and/or they forgot about him because he missed the season. This playoffs may have reminded everyone how good he can be, but the guy has a 128 point, award- sweeping season, could have won the Conn Smythe last year, and was 2nd in the league by a large margin in points and points per game to McDavid over the three seasons before this year.
And just because this playoffs might have a significant effect on the voting, that doesn't mean it's the sole factor anyone is using. If people are putting Kucherov over MacKinnon when they wouldn't have before the playoffs started, it means they likely considered them close before and this is just pushing Kucherov slightly ahead. This doesn't mean Kucherov therefore should be ahead of McDavid because he also had a better playoffs than him, because the gap between them for most people prior to the playoffs would have been much larger. Putting numbers to it, if before the playoffs, McDavid was seen as a 100, MacKinnon a 95 and Kucherov a 94, and after these playoffs McDavid falls to a 98, MacKinnon stays a 95 and Kucherov becomes a 96, it means the weight of the playoffs can push him ahead of MacKinnon but not McDavid.