Topmusicanswers
It would be to do with the way your diaphragm and thorax changes shape in a sitting position. I think you're right about the pressure. When sitting, the pressure of your thorax (chest cavity area) moves lower down perhaps producing this timbre you like the sound of.
Is it better to stand or sit while singing?
It is not necessary to stand while singing! Good and first for all, the real artist has to be able to sing in every position. And also in every situation. The key to perfect performance while for example sitting, is to keep your rib cage open.
Why is it easier to sing while standing?
Posture is off and cannot fill up with as much air. When standing your rib cage is naturally suspended -standing with correct singing posture lifts them even more. This allows your lungs to inflate and deflate with the least restriction. When sitting the rib cage rests on the lungs -even when sitting tall.
