how can MATH be this easy?

I can relate with your observation. Actually even i felt that the questions are quite easy. But then the problems in the OG are arranged according to their level of difficulty. So the last quarter of the sample questions are the ones that are more Real-GMAT-like if you are pretty good with maths.

Since the test is adaptive, most people who are above the 500 scoring level see more of the tougher problems in the real test which would resemble the last quater portion of the OG.

The questions on the OG are sorted on order of difficulty which means easy questions first and difficult questions last. I started practicing with OG and I had no problem solving problems 1 to 100 but I was not able to solve one single problem after the 100. So, I am now using some prep materials to help me answer question from 100 and above.