1 in 600 isn't anything to be worried about in particular and is not a high risk. 1 in 250 or 300 is where the screening test goes "positive" but that's an artificial threshold chosen simply because its similar to the risks of losing the baby to an amnio or cvs, not because 1 in 300 is noticeably riskier than 1 in 310... Only one third of one percent of babies with a positive screening result will have down's.

Your age related risk will be about the same or slightly "worse" than 1:600 which means your tests showed a pretty average or slightly better than average result to not change your age related risk much. Your previous result of 2500 was probably because your test results were much further away from an average result, to the lower risk side of things. There's nothing really of concern in the difference between the two results. Each baby and pregnancy is different and it may just be that this time your baby's results are much more average than your last one!