Why do computers,mobile phones and several other gadgets show different views

[Cybercheetah]

of the same page? Acording to what i know mobiles,iphones,laptops all recieve the information from the web servers right, so why are they interpreted differently and why are some things available on some platforms but disabled on others?
For example u cant group chat on facebook with an i phone or u cant view the low res videos in i phones