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

A few things that need to be considered to explain differences:

• By default, the SERP fetched by LowFruits is for mobile (not desktop), but you can change it in your default user settings here https://lowfruits.io/dashboard/defaults or the advanced options of your report (before generating the report).

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

• SERPs depend on the location and language. You might be doing your manual analysis in the UK and extracted the SERPs, using the tool, for the US.
• SERPs can change according to your Google search history. That's why navigating in private mode is important when doing manual analysis.
• Positions in the SERPs can change from one moment to another. This can also be seen in Google Search Console with daily position average data that isn't always round numbers.
• Results can depend on the Google Data Center.