There are so many variables that you haven't mentioned:-

1) Are the devices all of the same type (manufacturer and model)?
2) How many have you tested?
3) Are results reproducible?
4) Are cuff sizes all the same (and in good condition)?
5) Are the hoses all the same (condition, length)?
6) Are you concerned about pressure or BPM (or both)?
7) What sort of % variance are you experiencing?

Depending on which algorithms are used by various manufacturers, you may never get accurate dynamic results; but regardless of the tester being used, after testing a lot of NIBP units (of whatever type), and if you keep records, after some time you you will get a feel for when one or another results are outside "expected norms".

If you don't inspect ... don't expect.