I'll try to be brief here.
Firstly, the tests conducted for finger-prick versus venous blood are different. The blood taken from your vein has a different composition to your finger prick.
Fundamentally, your blood is dynamic, so the scores any test returns change, and it is possible to change quite a bit, in the space of 15, 30 or 60 minutes; moreso if you have recently eaten or drunk, but not exclusively.
And secondly, glucometers are allowed, by international standards to have a variance of c+/-15%, on any given test. Clearly that appears most evident if the numbers are higher.
So, our home tests are a fantastic, instant indication of where our levels are, but the lab tests are likely to be more accurate. (Lab tests will also have allowable variances, but I have no idea what they are.)
Does that help?