the gardening badges do not function like other badges. any others that i have looked at as you move up ranks you add another badge. with gardening they replace your prior one.
Re: why is there only one gardening badge showing?
It's set to function the same way as your "no title" level badge - so if you're a Magus myth wizard, it'll display the title "Magus Conjurer," and you can't use a previous badge from when you were, say, an Initiate. Same goes for PVP and pet derby badges as well. Completely speculation here, I could totally be wrong. But most other rank-based badges that you retain after leveling up were introduced after gardening -- the original rank badges (level, PVP, pet derby, and gardening) all function the same, with the exception of crafting. So I would assume KI decided after the fact that we might be interested in keeping lower-level badges, but chose to not alter the preexisting ones?