I mean according to the kit it states it produces 6 gallons of wine...but as mentioned, apparently that's 6 gallons
starting volume.
They don't clarify that in the instructions. 
Emphasis mine. Your statement is a fair statement in general, although the instructions vary by vendor.
Winexpert instructions specify the initial volume but never state the expectation after that. It does state that topping up is optional and should be done with compatible wine; water is not recommended. I would not expect a beginner to understand the nuances of this situation.
RJ Spagnols has online instructions that state to top up the carboy,
if required. So the final result will be 30 bottles, but it's not explained that the kit itself will produce less and the beginner doesn't realize they need to have extra wine on hand until they reach that step.
The real problem is that for a beginner, winemaking is a mystery and I expect most are unsure if it will work, which produces nervousness. Unless the expected output is stated clearly on the box and/or at the beginning of the instructions, anything that is mentioned will probably be missed unless explained clearly.
OTOH, FWK instructions specify 28-30 bottles is the normal result in several places, including the Minimum Equipment Section at the beginning, and the Bottling section. Overall, I like the FWK instructions far better than any of the vendors I've tried.
Although if all kit instructions were better, we'd have a LOT less traffic in the beginner's forum!
