Not really strange… depending on how much RAM you have it writes first at full speed to the RAM, while it writes constantly with full speed in the background to the disk . I guess you are confused (and you are not the only one) because the file manager just reports the transfer speed of DISK → RAM, but not RAM → DISK.
Dirty Pages are 10% of your RAM.
sudo sysctl -a | grep vm.dirty
You can check it live with iotop for example.
You can read more about it here: Decrease dirty bytes for more reliable USB transfer
Maybe it helps to understand the background a bit.