Reading Scores very unstable

So I took a couple of practice reading sat tests over the past few days, and my performance is crazy. The first one I got an 800. The second a 680, and the third a 790. Also, in the official sat I took in november I got a 650. I don’t understand what is going on. I would like to see consistent improvement, but this is all over the place. Did I really get the hang of the reading section and can get a 790 or was that just luck and I am actually near the 680-650 range?

Do you miss sentcoms? If so, then your experience is probably just an extreme version of a very common situation: until you know all the vocab, your score is extremely wobbly because some tests have words you know and others don’t.