That's what I figured too. Though that doesn't explain why Sap Sippers can snack on one sharp leaf/vine but not another.
Here's another odd thought. What's the limit to how many pokémon can fit in a ball? Catching exeggcute seems to involve catching six different bodies. Maybe they all are technically one pokémon, but that does raise the question of exeggutor's pokédex entry stating that a detached exeggutor head will revert to being an exeggcute, which implies each head is one exeggcute.
Even if that 'dex entry is just misleading (pokédex entries often are), there's also the case of slowbro and kangaskhan, which clearly consist of two separate pokémon that can fit into one ball. While having a dead part of a pokémon attached to a pokémon a la cubone could probably constitute as having equipment, which is confirmed to be allowed into a ball with a pokémon (alakazam, farfetch'd and machoke can bring their spoons, onions and belts into whatever ball they live in), the shellder attached to slowbro and kangaskhan's joey are clearly living beings separate from the main pokémon. Paras could be argued to be the same, as the fungi that grow on it are clearly alive and clearly separate entities from the insect. In paras's case, it's even possible that 3 separate beings can fit into one ball (though perhaps not, as the 2 mushroom caps could be part of one fungus, supported by parasect's only having one mushroom cap). And then there's vespiquen. A vespiquen's got six honeycomb cells, so likely one can carry up to 6 combee grubs at a time. And all those can fit in one ball. So there's at least one example of seven pokémon all being able to fit into a single monster ball.
So if there are cases of more than one pokémon fitting into a ball, in slowbro's case two pokémon of different species fitting into a ball, one has to question what exactly the boundaries are of how many pokémon can fit in one ball. If a slowpoke and shellder can fit in one ball, why couldn't a bouffalant with a joltik stuck to it fit in a single ball? If a kangaskhan and its joey can both fit in one ball, would shoving a psyduck into a chansey's pouch before catching the chansey let you catch both with one ball? It really does raise questions.
Hm reminds me of a funny little comic where a trainer catches a bunch of things in one pokeball and what came out was a horrible monstrosity of fused together pokemon.
Hm perhaps what pervents you from catching mulitple things is a safety device that the manufacters put into the ball, weird cases like slowbro have like exception programmed in.