Hi Economist2015,
The scoring algorithm on the Official GMAT is far more complicated than most people realize. It takes into account a number of different factors, including the relative difficulty of the question, whether you were expected to get it correct or not, the placement of the question, what's going on "around it", the "strings" of correct and incorrect answers, whether the question even counts or not (some questions are "experimental" and are worth 0), if you leave questions unanswered and incur a penalty, etc. As such, you shouldn't be spending time trying to figure it all out. You'd be better served working on building up your skills.
Since the scoring algorithm on the Official GMAT is proprietary, no GMAT company has an exact match for it, thus CAT scores can vary based on the 'biases' involved in their respective designs. You might want to use a different set of CATs as a comparison for how accurate these scores might be (especially the 2 CATs from
www.mba.com).
GMAT assassins aren't born, they're made,
Rich