When I check manually the SERPs, I get different results ?

A few things :

1. By default, the SERP fetched by the tool is for mobile (not desktop) but you can change it in your default user settings or the advanced options of your report (prior to generating the report).

Results can also depend on the type of device and browser too. 

2. Positions in the SERPs can change from one moment to the other

3. Results depend on the Google data center.