Howcome when a being have gas, sometimes it comes out as a burp and other times as a fart?

Burps are the result of swallowed gas, or gas evolved in the stomach.
Farts are the result of gas generate during the digestion process in the intestines, usually a by-product of bacterial handling on the food consumed.
Two different things that share the same autograph.
Burp= comes up if gas is trapped in the Upper GI tract.
Fart (flatulance) = comes out if gas is trapped within the Lower GI tract.
that's funny
Depends on whether the gas is in the stomach or life-size bowel.