I have found the kaplan tests to be a bit harder and not as consistent
My gmat baseline was 650 46q 37v with absolutely no studying
after a month of studying did 1st kaplan 630 q44 v34, it felt a lot harder than the gmat prep
2nd one was after studying quant for a month straight and going through the whole quant section in two books, kaplan and og.
I got a 680 q49 v33, this felt good because i had significantly improved my quant to where i wanted it to be and I didn't expect the verbal to go up much because i haven't really started to study it.
Just took 3rd one today and got a 610! q41 v31, it felt incredibly difficult compared to the other tests, and I was no less prepared, actually more so because i have been studying verbal for the last week