Re: bridge/intonation
In a very cramped nutshell: because the stretching of the string caused by pulling it off axis (i.e. fretting it) increases the tension on the string less on the treble side than it does on the bass side. There are quite a few other factors that play into it though....
I´m not a physicist, but if you want to build guitars you also gotta learn this kind of stuff, but it´s not really something that can be explained in detail within the confines of an Internet forum IMO...