I assumed Kyubey was the name all Incubators used so that even if they end up in different locations, they still appear to be just one single entity. Referring to themselves as a species, or otherwise leaving hints that suggest there are more than one, makes it easier for prospective magical girls to realise that there's some kind of agenda going on. Which there is. One that would make most of them less likely to agree to the contract.
I was thinking that earlier today, (As opposed to last night my timezone) but I like to think they all spell it differently.
Like: Kyubey, QB, Cuebee, Qubie, Qby, etc.