I encountered similar situations when I was preparing for my GMAT. My Kaplan
scores were significantly lower than those for Powerpre (500/600 vs 750+-). My actual score is 700+.
Personally, I feel that the Kaplan
tests are not that good. Not because of the low scores, but because the questions were not well put. e.g. there were suspected missing lines in the RC.
However, it is useful to take the Kaplan
tests to "season" your mind and build up the mental stamina. But ignore the scores.
Also, I suggest the last full diagnostic test should be taken about 2 or 3 days before your test. Leave the last few days to rest your mind abit. Just lightly go thru the OG, redo those questions that you got wrong before, etc.
Hope this help.