I'd steer away from thinking the instrument is inaccurate.
More likely inaccuracy in the tests done at doctors or whatever.
Reason being, conditions vary dramatically at, or on the way to the latter.
Medical staff used to argue with me about my readings so I was advise by my GP to do a series of tests around similar times, night and morning, for a week and look at the average.
Also told me to keep the results and show anyone who asked.
Around a week after, had the need to visit the outpatients section of local hospital, waiting room for 5 hours on hard as seating and freezing air conditioning.
Issue came up, showed them the previous test result sheet, .............oh...mmmm.....ok....no problem.
Well, there was, no hospital blankets given out anymore in the freezing air con because "someone might trip over it" ...........crazy shit these days.
Overall, Omron BP tester should be fine.........maybe you need new batteries.