No, it is not unknown. You can use the same finger and two different strips will definately give two different results. There are many influencing factors about finger testing, all of which will alter the results.
I'm exactly the same! The other night I was 22 in one hand and 15 in the other! That's a massive difference and my hands were definitely clean! It's so frustrating cos if I injected to cover the 22 I would of gone low if my blood was really 15! So confusing x
There's a link in there explaining why meters vary so much. In the box of test strips there should be some literature explaining the precision of the meter.