I think it also comes down to the fact that staff in pubs (not the managers) tend to be very young.
To many (I was one once) it’s the first job you get while on a gap year or at university. And, as several have highlighted already, the training bar staff get is often minimal. You generally have a trial shift at which you are shown where everything is, told what the general duties are (which generally includes food service, clearing tables, tills and such as well as drinks) and probably will be given a quick chance to pull a pint. These are kids really trying to just about cope.
Not only have they had minimal training - they certainly are not professional mixologists or sommeliers (both of which takes years of training and experience and is the drinks equivalent to being a high end chef) - but many are about 18 and haven’t had very much experience of drinking themselves.
So they’ve neither the experience professionally or socially. And that’s before people start throwing in their own particular names for certain drinks!
I learned far more from the customers at the bars and pubs I worked at than from the managerial staff.
Now it can be very irritating, but at the same time it’s been this way for decades now and it’s only going to get worse in an industry under such strain. Certainly, as often in these sorts of situations, while these things can be incredibly frustrating, the fault, if any, sits far further up the chain of command than the poor sods at the coalface, on the floor, on minimum wage, poorly trained and having to deal with irritated customers and a very fast paced environment.
I think what you and your husband did was absolutely right, explaining it and being nice about it. She’ll (hopefully) remember what a shandy is now and has learned something. The real issue is when customers come into these places and treat staff like badly behaved servants who should somehow have an encyclopaedic knowledge both of every possible drink anyone might ever order and of the regional and particular names people use for them.