OK, tried a couple of things including quantum change, qlen, nothing really changes things, but if I set rate to 950000, then we get 50000 more, with the same pings etc.
Only explanation for that is, that there is a bug in some calculation.
http://www.desipro.de/openwrt/flent/tcp_nup-2021-11-08T004821.799927.BURST112500.flent.gz