Slowbro or Slowking

I think that's the reason. Slowbro's charge moves are better for defense then Slowking's. Hence the better rating for Slowbro.

yea i couldnt agree more. slowking is more of an offensive pokemon. slowbro better on defense, slowking having fire blast is interesting of course, but even though the move hurts grass he gets no STAB from it and the move doesnt hit frequent enough on defense. same story for blizzard, tho its a better move than fire blast