WHy

is it that when im in a car traveling at some speed and i observe another car traveling at my speed plus some arbitrary amount (lets say +5mph) i observe that cars motion at less than the actual difference in speed between my car and it.

tl;dr

why do cars going alot faster than me on the highway look like they are going maybe 3-4mph past my window

After you've been traveling at high speed for a while, you start perceiving speeds as lower than they actually are. This is a well known hazard of driving, but I don't know its name in English.
And how often do you have cars driving past at 10mph for comparison? I'm sure it would be easier to assess if they'd drive in sensible metric units.