Cat communication is a funny thing, isn't it? Our kittens weren't vocal at first - they were semi-ferals - but then Paddington found his voice. He only chirps and chatters! The other kittens have picked it up. Seriously - anytime a normal cat would meow or anything is when they do their little chirps. They also use it to apparently 'talk' to each other. One will be downstairs with us, and one will call from upstairs, and the downstairs cat will run upstairs, chirping and chattering away. I usually then find them upstairs hunting a moth or something...It is adorable. I think all cats just communicate in their own way.
My husband's old family cat - Rosie - used to do the same. She never really mewed just chirped. Her son, Spike, however, had the biggest meow you have ever heard!