Got shocked, scored different from my practice tests

I took the GMAT exam 10 days ago for the second time and got 550(Q48,19). Actually the first time I did not have any idea about what the GMAT is! I got 540(Q48 V18) in Jan 2009. For the second time, I studied for three and a half months. I studied Manhattan Sc, OG 11 and 12 and reviewed OG's for 3 t...