It's not per 90 min. It's per appearance. You need to adjust it to get to per 90 min played.
So just going through Richarlison's possession lost stats per game in the PL this season:
Woolwich: 71 mins / 10 lost
Leiceseter: 59 mins / 11 lost
Fulham: 90 mins / 14 lost
West Ham: 14 mins / 2 lost
Forest: 16 mins / 5 lost
Wolves: 14 mins / 5 lost
Chelsea: 33 mins / 8 lost
Total: 297 Mins / 55 possessions lost
Per game: 42 mins / 7.9 possessions lost
Per 90 : 90 mins / 16.8 possession lost
Son is at 14.6 possessions lost per 90 this season.
Even at Everton, he was never known to be a very good at ball retention. In fact, he's had years significantly worse in terms of ball retention than Son is having at the present moment. A couple seasons ago, he was at 19 possessions lost per 90 mins.
Ah OK.
But to balance that up, Richarlison (47.9) sees more of the ball per 90 than Son(41). carries the ball more than Son per 90 (34.8 v 26.7) he also works harder (almost double the pressures (20 v 11) and tackles 1.8 v 0.9) per 90.
This is the conundrum. Son is capable of scoring great goals, but he's normally incredibly patchy. When he's not scoring his all round contribution as absolutely gash. He's fucking atrocious without the ball, rarely gets within two yards of pressing anyone, cannot hold the ball with his back to a defender without coughing it up in panic and his passing, even over short distances can be really erratic. And he's non existent in the air anywhere on the pitch.
Richarlison is also far from perfect, bumbles, and is less likely to score than Son, but will work harder for the team and more effectively, much better without the ball, and will put himself about a bit more for his team.
I think until we have a genuine upgrade on both, they are going to share minutes. I quite like Richarlison starting and wearing defenders down and then bringing Son on to go at tired players. That's what happened against Leicester.
Your point about us being better after Son came on is a bit disingenuous. By then Leicester are a goal down and chasing the game, and getting desperate throwing more people forward. But it's not like our overall performance improved, there were just more spaces on the counter then. Son's first goal he takes on a worldy shot instead of playing Hojbjerg clean through. Really dubious percentage call for the team - but it worked out for him this time.
And what about all the games Son started at the start of the season where he was awful - and the negative impact that had on our performances? Against Marseille there was a reverse scenario where Richarlison came on and won the game with two goals.
As I said, it's a conundrum, because both are capable of decent things. Neither are complete.