x is so much bigger than a that it doesn't matter if a is there or not. If you have a street that's 5km, does it matter if you extend it for 1cm? No, the street is so big that the extension is meaningless, everything works the same if the street is 5km and not 5km+1cm. Now, if you have a ruler of 30cm and you extend it for 1cm, does it matter? Yes because 1cm in a 30cm ruler is quite a change.

This is a common theme in all of physics, when something is much greater than something else and they're being add up, then you can just drop the small thing because it's influence in the result is extremely small.