smartconnect RT-AX88U and repeater RT-AC66U works better then expected but how?

i am recently playing around with smart connect and it works better then expected

2.4Ghz and 5Ghz have the same SSID
2.4Ghz on channel 9/up
5 Ghz on auto

i can walk around the whole house doing a transfer test and it switches without interruptions back and forth
on 5Ghz i get around 600Mbps and on 2.4Ghz around 0-180Mbps
when i get to the second floor the transferrate drops to 4Mbps because it is almost out of range.

now the weird stuff.
i have added a RT-AC66U as a repeater to the...

smartconnect RT-AX88U and repeater RT-AC66U works better then expected but how?