If both of them are presented in front of you, you lift one up and weight the other, it's immediate that the MBA is subjectively lighter.
But if one purchased the MBP without having touched the MBA, the weight is actually a non-issue. Its not an everyday occurrence to have access to both that it would make a true difference.
Just like how the display is very very close that it could be negligible, as to your point, the weight could be negligible too.