This happened to me with GoDaddy and Namecheap before, which is why I switched to using Gandi for all my domain searches... Now I'm regretting it!
But as @Jasper_ said, this could be a problem with the domain name registry selling/leaking that info (AKA all their 'is_available' queries), and not the registrar.
But as @Jasper_ said, this could be a problem with the domain name registry selling/leaking that info (AKA all their 'is_available' queries), and not the registrar.