I am genuinely perplexed as to why you think that's odd.
To classify a population of plants as a distinct species, subspecies, or variety, they have to be genetically isolated from other populations, either because they are incapable of interbreeding with them (species) or they are geographically isolated from populations they would otherwise be able to breed with (subspecies and varieties). At least that's the idealized version of how it works, reality is often quite a bit messier. But that's the general idea.
The fact that some individual plants of Maranta leuconeura look rather different than others is really of no consequence unless at least one of the above conditions is met.
I'm not a plant taxonomist, nor do I know anything about the genus Maranta. But, maybe it would help to think of it this way. Are blue eyed people a separate species or even subspecies from green eyed people? Obviously not, and the reason why is that they are not genetically isolated from each other (they can and do interbreed). Apparently, it's the same situation with the red veined Marantas.
