I find that the readings from my wrist-cuff monitor are very similar to those that I get from upper arm monitors at hospital clinic sessions and at my GP's surgery.
As Sqad frequently points out, BP readings are only a snapshot of your blood pressure at a particular time. If your BP is, say, 140/85 when you take a reading (by whatever means), it could well be 130/80 or 145/88 just a few minutes later. So even if a BP monitor is inaccurate by a few percentage points, it's not really going to make much difference.
Medical professionals are unlikely to be interested in precise figures anyway. To them, readings of 120/80, 115/78 and 124/83 all say exactly the same thing. (i.e. 'Normal'). Similarly readings of 140/85 and 145/88 are likely to interpreted in similar ways. (i.e. 'A bit above where they should ideally be, and probably worth trying to bring them down a bit, but not immediately alarming').
So there's no real need for home BP monitors to render absolutely precise readings.