OG12 & MGMAT strategy (quants ) guides and not sufficient ?

Hello Everyone ,

So after not figuring out my own i decided to share my problem on this forum so i can get help from the experts to help me out and letting me know wheres the problem or where i am lacking.
After doing MGMAT Quant books i started doing OG 12 (could solve most of the question in time and my accuracy is good)and while solving the OG i felt confident so, to check my progress I took GMATPrep and MGMAT Test and in these two tests i find difficulty in solving the Quantitative problem ( taking to much time , not getting the final answer). So i got depressed and then again i started doing OG 12 with no additional study again I can solve OG 12 question with ease and with very high accuracy in time.
So anyone can tell why i am not able to perform well on GMAT Prep and MGMAT test while i can perfrom well on OG 12 ( in timed environment) . Are the questions in OG 12 are very easy in comparison to GMAT prep and MGMAT Test ? Any suggestions to improve my performance on GMAT Prep /MGMAT Tests just as on OG 12

PS : Are there any books only for questions (no theory like 5lb Manhattan GRE book)


Thanks,
Mohit