There could be, but it’s impossible to know for sure if this was intentional or just an error.
Enhancement bonuses are “weaker” than other types of bonus, because they are so common (and typed bonuses don’t stack). Someone with a belt of mighty constitution sees no benefit from plant shape I’s Constitution bonus, but does from that of plant shape II and III. That could be intentional, a way in which the higher-level spells are more powerful. And plant shape I does apply a size bonus to Strength in the same line it applies an enhancement bonus to Constitution, so you could argue that it’s not like they “forgot” to use size bonuses or something.
On the other hand, it is weird, and other polymorph effects don’t seem to do that—they just give size bonuses to Constitution. Moreover, it’s very easy to imagine someone saying “oh those are supposed to be size bonuses,” and fixing the Strength bonus—the first one on that line—and then moving on to further lines without scanning to the end, and so missing the enhancement bonus to Constitution.
So really it could go either way. It’s certainly playable/sensible as written, and there might be reasons to do that intentionally, but it’s still weird and unusual, and looks like it might very plausibly be a mistake. There’s not really any way to know for sure. As a 5th-level spell, though, I definitely rule that plant shape I gives a size bonus to Constitution—even if they intended the enhancement thing as a drawback of the lower-level spell, I don’t think it’s deserved.