I guess in a roundabout way its about the age of the meat.
If its frozen, there is no way telling how old it is, and there used to be rumors floating that companies Mc ... King bought some kind of cheap government beef, that was frozen for decades .... and keeping something reliably frozen for years can be tricky and if the government is selling it, it must not be fit for soldiers to eat...

OTOH, if the meat (or lettuce or tomatoes) are merely refrigerated, then they last at most a week (?), so you know they're really fresh, and not 10 years old

That's the only reason that springs to mind, but remember, advertisers are notoriously stupid and liars, what they say doesn't have to be true or make sense, it just has to get you to buy