Beat The GMAT a GMAT & MBA community

These are my Manhattan GMAT Practice Test Scores :

1.MGMAT CAT 1 - 720 ( Q47,V41)
2.MGMAT CAT 2 - 700 ( Q47,39)
3.MGMAT CAT 3 - 660 ( Q43,V36)
4.MGMAT CAT 4 - 710 (Q 48,V37)
5.MGMAT CAT 5 - 760 ( Q 49 , V45)

There seems to be a huge variation between MGMAT CAT 3 and MGMAT CAT 5 , what could be the reason for this ? The only thing I've changed between the two tests is that I've implemented a timing grid strategy where in I just give up on a question if I've spent more than 3 minutes on it . In CAT 3 , for many questions I used up like 5-6 minutes .