I don't know how serpchecker defines a position, but in Google Search Console the position value shown in the Search Analytics report is the position of the topmost link to your site or page in search results, averaged across all queries in which your site appeared. So, for example:
1. If one query returned your site at positions 2, 4, and 6, its position is counted as 2 (the topmost position).
2. If a second query returned your site at positions 3, 5, and 9, its position is counted as 3 (the topmost position).
3. The average position across these two queries is (2 + 3)/2 = 2.5.
As you can guess, there could be significant differences in position reporting, depending on the method used.
If you count in the fact that everybody's results are nowadays personalized (and localized), the differences between tools can be even bigger.
Personally, I have no reason not to trust Google Search Console data.