The canonical answer is that Jon is reacting to Garfield and Odie’s actions, not their thoughts: https://scifi.stackexchange.com/a/222029

Jim: I wanted to ride that real fine line between wanting to understand what the cat says, I think people honestly feel that cats think in English, that cats understand everything as they do but obviously they don’t talk so I wanted to ride that line between here’s what your cat is really thinking to give pet owners insight into what their pets are really thinking without giving it away, to have him really talk and Jon understand him, would have made it more of a fantasy. I always wanted to create a strip that could have cats, if they had opposable thumbs, would drink coffee if they could. Apparently Heathcliff, a cat in a comic strip panel, would fly planes and drive cars and be on a desert island and I didn’t want to do that. I wanted to create a very real world and then go out to the fantasy from there. And, you’ll notice that when Jon understands what Garfield is saying, it’s something Garfield does with an action, not a word that communicates it.

Found someone that removed the thought bubbles from a few comics as an example:

https://bogleech.tumblr.com/post/76445444351/i-know-garfield-minus-garfield-was-the-one-that

On the other hand, there’s strips like this one that seem to have Jon hearing Garfield’s thoughts, though that can be chalked up to Jon making up dialogue for his pets to amuse himself:

https://garfield.fandom.com/wiki/Garfield,_January_1992_comic_strips?file=1992-01-21.gif

Here’s another one:

https://garfield.fandom.com/wiki/Garfield,_May_1986_comic_strips?file=1986-05-20.gif