It doesn't involve the same range of products.
In a grocery store, there are between 25,000 and 30,000 products. I don't think that is the case in every restaurant.
As well, each product is different. I can find you 25 different cost prices for chicken breasts. Agricultural producers in that field are well aware that there are different qualities and different processes. Sometimes people think it is the same product, when it is not the same.
Personally, in fact, I tend to hear the opposite. As a consumer, at least, I find that my restaurant bill is climbing faster than my grocery bill. That is a personal comment, however.