Edexcel new specification chapter two

Hello,

I am studying A Level Edecxel with the new specification.

I was solving the problems in chapter two, and I got some of the questions wrong because of rounding problem.

Why some of the answers are rounded while some of the answers are not rounded?

I am so confused.

For example, I got 18.6 for an answer so I wrote down 19, but the answer was 18.
But on another question, I got 11.5 as an answer so I wrote 11 this time (in the book, most of the answers are not rounded), but the answer was 12.

Is there anyone whose experiencing the same problem?