It's basic grammar. You don't have 'less than' 10 items, you have fewer than 10. Same as you don't ask for fewer coffee in the cup so you can add milk, you'd ask for less.
Math a. has different rules than language, and b. inequalities are generally of indiscriminate quality. If you're using a variable, there's not a way to know what the variable represents, etc.
If you don't understand that 8 items is less than 10 times, you have a problem. I don't go to the grocery store to learn grammar, and from everything I have read, this statement , especially in grocery stores, in commonly accepted now, not in England, but here in the US.
And when my kids were small they were taught that 8<10. No variable there. So, it stands that 8 items are less than 10 items. If that makes me wrong so be it, I have better things to worry about.
Also to me it is about word placement, just as you wouldn't end a sentence in a preposition I find that saying 10 items or fewer just sounds so wrong, now if the sign stated fewer than 10 items I would get that. Just as I would say I have fewer items than you, and not I have less items than you. But the other just sound plain stupid. Kind of like "Where are you at?" Grates on the nerves.