Why does store bought tequila taste different that bar tequila?

I’m not much of a drinker but I drink sometimes, and before this year I only drank store bought stuff. Today I tried my tequila (pepe lopez premium gold, been in the cupboard at least 3 years) and it tastes super different, what could be the reason?

Bar tequila (shots) is much milder and sweeter, while pepe lopez is very bitter and strong? is it like that with all store bought? I didn’t think about this before because thought going out drinking is a waste of money lol