The difference is in whether you are testing whole blood or plasma levels for glucose. Most meeters test whole blood while most labs test plasma. The whole blood reading will generally be about 12% lower than the plasma reading, so to compare against the lab when it is testing plasma and you are testing whole blood, multiply your meeter reading (if you meeter tests whole blood) by 1.12. Divide by 1.12 to go from plasma to whole blood readings. Readings can also vary between labs and with the technique of the lab technician.
Venous blood versus capillary blood also makes a subtle difference as well as whether the sample is taken from the fingertip or another part of the body such as the forearm. Figertip samples are best when you suspect that your blood sugar is dropping rapidly as this will show up there first.
Blood glucose meeters can be off by as much as 20%. The best thing is to use one meeter because that will at least give you the consistent trend for that meeter. The whole idea behind blood glucose meeters isn't to give you an absolutely accurate measurement, but to help you spot trends and determine if your blood sugar is rising or falling quickly.
The lab readings are usually far more accurate so if, after adjusting for the difference between whole blood and plasma readings, your meeter is close to what the lab readings are, you've got a pretty accurate one.
As for the A1C differing from your fasting and 2 hour readings, what your testing isn't showing is how high your blood sugar is going after you eat...it's showing how close to normal it is 2 hours later and when you haven't eaten for several hours, not what it does all those times in between. Given all that, I'd say that an A1C of 5.5 shows that you are doing pretty well!