trying on shoes

That's a normal situation, which is why you should not shoe shop at certain times, unless you know your size in a particular brand with good quality control. A shoe tree will help keep the shape and expand the left foot a bit, just put a thick sock on the shoe tree when you put it in the shoe, and it will give a little stretch to help the shoe fit better initially.

Often they put the right shoe on display so that you can try it on, and after that they bring you the left shoe. The right one has just been worn more often and is therefore a bit more stretched

Practically everyone's feet aren't exactly the same size. Just for some people it's more significant than others