As convoluted as most WTS doctrines are, this is one of the more convoluted. Jesus is ransom for all followers, but mediates only for the anointed. The anointed make up the Christian congregation; the rest are just supporters of this heavenly class of spiritual brothers to Christ.
This confusing teaching is why on one hand, you could argue that JWs (non-anointed) are not true Christians. They're just supporters of the anointed, and followers of Christ, but not technically members of their club.
On the other hand, the WTS never makes this definitive statement, but purposely leaves it obtuse. JWs consider themselves Christians based on their acceptance of Christ as their ransomer and leader. On that basis, then yea, they can be considered Christians.
I think this goes back to the early days of the WTS when they expected Armageddon early on, with only a limited number of adherents and survivors. They outgrew that expectation, and had to rewrite doctrine. But in true WTS style, they created a bigger mess than what they already had.