The CV isn’t trained on observations that have a genus-level ID once any species in the genus is in the model. This means that if the only species in the model is atypical of the genus, it will have trouble recognizing the genus.
The CV can suggest broader taxa because it has some information about taxonomic hierarchies. But it is doing this only using the material in its training set (i.e., photos representing certain species only, not the genus as a whole) – it doesn’t know the genus separately from the species within that genus. The CV is based on pattern recognition only; it doesn’t actually “know” what is in any of the photos it is trained on – that is, a spider is merely a particular combination of shape and color against a larger background of other shapes and colors, it can’t suggest IDs based on morphology except insofar as these differences represent consistent features in particular sets of its training material.
I wrote a longer explanation here: https://forum.inaturalist.org/t/allow-some-non-leaf-taxa-to-be-added-to-the-cv-model/63937/9