what turntables are you using? how much bpm difference is there to destroy your mix?
the rotation of a turntable us not always 100% being mechanical there is something called wow and flutter. this is why the bpm has fluctuations. however this should be only small (give or take .5 bpm). this is why you still need to nudge and drag when mixing vinyl.
cheap turntables have weak motors and will differ greatly. super oem or technics are what you need for motor stabilisation.
when you play in internal mode I bet the bpm is solid.