DP and his ex always work out between them the 'real' cost of DSS, and then he pays her half of that.
So when DSS was little and in childcare, DP's payments were higher because he was paying towards that. Now he's in school and childcare costs are much lower, I believe DP pays less (even though he pays for half of clubs and things, it still works out cheaper)
I've no idea if DP pays more or less than the CSA amount, to be honest. He is a fairly high earner, so it's possible that it's less - but then his ex is a high earner too. She earns less than DP, but her joint income with her DH is more than DP and my joint income...plus her mum does a lot of the childcare for free, but should DP be paying as if that cost something? Even though he doesn't have the option to move close to his parents to help with childcare because that would mean moving away from DSS...there's a lot of Catch 22 in there...
They are able to work out payments like this because they both trust each other, but this is obviously rare and I think the CSA system is just a next-best solution, but there are so many problems with it. Plus it's compounded by the fact that it is inevitably more expensive to raise a child across two households than it is to raise it within one household.
In brief...it's never simple, it's a minefield of working out what's fair, but in OP's case it does seem like more than enough money! And even if her DP is a really high earner, he's already paying really high maintenance and his kids will be benefitting from his high earnings during their periods staying with him.