This behavior is caused by the "Enhance Pointer Precision" mouse setting on Windows, which applies acceleration to cursor movement.  The reason that it behaves differently on Bluetooth compared to SwiftLink/USB comes down to polling rates. 

Bluetooth operates at 133Hz, compared to the 1000Hz of SwiftLink and USB. So even when you are moving the mouse at the same speed, on average Windows sees almost 8 times the amount of movement per mouse report over Bluetooth. Which results in heavier acceleration being applied, increasing overall cursor speed.

So this issue can be resolved by disabling Enhance Pointer Precision from Windows mouse settings.

Since the issue is caused by polling rate, an alternative solution would be to reduce your polling rate in the X1 Control Panel to 125Hz (to closer match Bluetooth's 133Hz). This isn't recommend as it significantly reduces the performance of the mouse. But if you want to keep Enhance Pointer Precision enabled it is an option.