best check responsive service ?

hi

Im working on a theme and trying to work out the responsive side.
however, when i check those “is it responsive” sites i got diffrent results !!!!!!!

it’s very important that i don’t skip one single pixel otherwize the hole layout will fall apart

i used these two
https://mattkersley.com/responsive/
https://www.isresponsive.com/
but got diffrent result ??
https://i.imgur.com/AYH8ygt.jpg

what site/service do you recommend i should rely on to check if my theme is build for all devices ?
thank you in advance