It gets better is the same as saying it becomes better. The offer is good but it becomes better when you know these additional details. It's the same as the difference between saying "the dog barks" and "the dog is barking." The dog barks because that's just what it does. The dog is barking right now. If you say it is getting better that would mean it is becoming better right now rather than the offer gets better in general.

The first example has a positive sense. "This is a good deal, but it gets better." Translation: "This is a good deal, but there is something more that is good about this that I haven't told you about yet."

The implied tense here is future A clearer sentence would be "It will get even better [very soon]" It is usually used two ways: To say that there is something better about a situation that you probably didn't know about. or To say that a situation is even worse. In this case the mood would be sarcastic. "I had a flat tire on the way here." "That's too bad." "No, wait! It gets even better. My spare tire was flat, too!"

"It gets even better" can be used in an ironic sense.