Compensating clock drift "by hand" - what have I solved?
Sequencer and soundcard clocks come from crystal oscillators. 

8.4ms/min is a 140ppm difference in frequency, which is a bit high but not impossible, for example if both use 100ppm accuracy crystals and you get a bit unlucky.

It probably depends on temperature too, so if the temperatures of the sequencer and the USB soundcard inside the mixer vary, your "magic" -8.4ms/min may change.