[[Mod. note -- One plausible reason is that stars change in spectral
type as they age. Wikipedia gives Tau Ceti's age as 5.8 Gyr, Alpha Cen B
as 5.3 Gyr. The two stars also have somewhat different metallicities
(which affect spectral types).
Note that Tau Ceti is G8, while the Sun is G2. After G comes K, so the difference between G8 and K isn't that large. There are different metallicities, and also Tau Ceti is a single star, so there is a
In this case, both are main-sequence stars. However, the spectral type
itself says little about the mass; red giants and red dwarfs, for
example, can have the same spectral class (but different luminosity