I got similar scores on the Kaplan practice and the real DAT.
I have found that many who say that their real scores were lower are talking about their very best test. For instance if you took six practice tests and got a 16, 19, 17, 22, 19, 20, and then took the real test and got a 19 you would say you did better on the practice tests, when in reality you scored right around your average.
If you've taken all the Kaplan tests and been all over the place then your score might be all over the place, but if you are consistently hitting 19's and 20's for example, you will generally score in that range, that's true from my experience and a from a number of friends.