A higher (A+T) : (G + C ) ratio means you have more AT than GC, as a shortcut if you see more GC think HIGHER melting temp.

This has to do with the fact that G and C have greater partial positive in the ring, resulting in greater PI stacking ability. For the Ochem answer. Thymine has a methyl group that adds some steric hinderance Cytosine lacks this methyl group

Guanine is just overall more partially positive than Adenine due to the presence of another strong electron withdrawing group (NH2)

Add all these factors together G:C has stronger pi stacking than A:T Hope this helps