Why does Joining objects together stretch UV mapping for some individual faces? I've seem to encounter some kind of anomaly in my model. I have created a model of a car, composed out of multiple meshes. After joining the pieces together, i began texturing the model, however, the UV maps of certain parts are stretched (but cleanly unwrapped) and distorted in a certain direction, while some unwrap normally. 

I've nailed the problem down to where it seems only the base, or mesh i began with unwraps normally, while the other meshes i joined onto it have bad unwraps. I've tried unwrapping individual faces and get the same result, even after marking seams 

Here is an example


What is causing this?
I've figured it out, Selecting the object and pressing CTRL+A in 'Object Mode' then applying  the scale fixed it.

(credit to @Paul Gonet)