Why K3 remained a battlecruiser

What made the K3 a battlecruiser rather than a fast battleship?

Its armour coverage and subdivision were less comprehensive than a battleship's, despite its heavy guns and high speed.

K3 remained a battlecruiser because maximum speed and gun calibre did not define the type by themselves. The decisive features were how much of the hull received heavy armour and how the internal subdivision supported survival after damage. A ship could carry battleship-scale weapons while accepting reduced protected volume to gain speed. Armour coverage and subdivision provide a structural definition beyond labels or headline specifications.