I have seasonic m12II 620w psu and my case has space for putting them bottom of the case. There seems to be a dust-free filter at bottom rear and i montaged like psu fan looks at bottom. should it look up or down?
It's normal to have different voltages reported.
Neither one might be right. But BIOS is usually more accurate.
Your fan is drawing air into the PSU and exhausting it out the rear.
As long as your case doesn't sit on carpet and it doesn't collect too much dust on the filter you'll be OK with that setup. Check the filter pretty often, at least once a month, till get you a feel how often you need to check.