I believe it shows the default as whatever the atk/hp is has after it ate something, and when damaged, the current hp will go down and the default will show how much it should be. that's how it works for creatures with the growth ability.
That's a lot more useful, but maybe "default" isn't the right word here? Default sounds like it would refer to the stats that appear on the creature's card, which would be redundant as the card still appears when you mouseover. What this number really represents is the total of the creature's card stats and any permanent effects. (Plate Armor, Mutation, Growth, etc.) It might be better to call those the "Base" stats instead, to avoid confusion.
Having played around with it in the trainer, I really like the way the info panel works. I'm curious about the Delay section, though. I don't think anything currently produces that effect for longer than one turn, so is this an indication that we may see more cards that put things in time bubbles?