thanks a lot @lucas and @RaptorUK for your advises
i change my printer and it turns out right
the 20cm ref line was 19.3cm but after changing my printer the measurements were all fine
thank the rest of you guys for your suggestions but the footprint is just fine
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And don’t forget, there are connectors that have a pitch of exactly 2,500 mm. and other that have exactly 2,540. So a 14 pin “EU-style” connector has exactly 35mm. between pin 1 and 14 and an “Imperial” style has 35,56mm. … and you can’t force these connectors in one another’s holes! 
Bloody Imperial style… but Kicad can handle this quite well! Even Thou/Mills 
But I suppose everybody knows?
I used to have this problem when I was a student, and I used to print my PCBs on tissue paper.
The problem is with the printer scale, not the EDA software.
You just need to lay out the PCB, then send the Gerber file to a standard PCB manufacturer.
That can be the issue, but not in this particular case. Look at the screenshots of the EDA data: 50.92mm versus 52.8mm for the big green connector.