Deep sleep Ultra Watch inaccurate

My Apple Ultra Watch appears to be miles off on my sleep data in particular deep sleep tracking. In addition to the Apple Watch I have been wearing on my other wrist a far cheaper, supposedly inferior smart watch that tracks my deep sleep much better if I was only getting between 4-16 mins deep sleep each night I would be dead by now! Rather miffed as I did not spend all that money on an Apple Ultra for it to produce inaccurate data…………Any suggestions please thanks 🙏