That is pretty interesting. The way I see it, there was probably less competition in terms of new releases for Saint's Row on the Xbox platforms compared to it on PS platforms, so it had a better "spotlight" so to speak. on Xbox where less games to compete with drew people's attention to Saint's Row.
At least that's one theory. The other two are that either more of the Saints Row audience are on Xbox (I know the original games were more popular on 360 than PS3, in fact the first few games were exclusive to 360 for a long time it seems like. I don't even remember when Gat Out Of Hell came out for PS4 (I think it skipped PS3?), but I did play that on 360. I know it's been a long time since a new Saints Row, but there's precedent more of that game's audience could be on Xbox console-wise.
The only other explanation is that Saints Row was one of the games available to pick up for 'free' with Series S consoles. Elden Ring was one of the other games. Since those aren't technically hard bundles I think the game would be counted as part of the sales, it's just there's probably a voucher at check-out to write off the cost of the game once it's been rung up as purchased, or something like that. MS started that promotion at least since the middle of August, it's even here in the U.S so I wouldn't be surprised if there's a similar one in the U.K.
Don't worry, I'm sure Sony have their holiday supply ready to go for Fall and Winter. The important thing is they've seen strong enough increases in stock to start making up for paltry supply earlier this year, and it's only getting better.
Even Series X is easily available there? Dang; I know Series S units are, those are pretty easily available everywhere tbh. But I'm guessing in some spots Series X unit supply has started stabilizing but the demand could be tapering off.
Time will tell of course, as always.
Series X has been fairly easy to get for a long time here in Ireland.