Quote Originally Posted by Ruck View Post
And I'm now realizing I don't know exactly how to interpret it. (A Protean also has Strength of 53, for what that's worth.)
The "base attack" effectively refers to the pure "accuracy" of any attacks the Protean's would make before its strength or dexterity are taken into account; the Grapple entry is that same base attack value plus the modifier for its size. The specific entry below for the slam attack factors in the Protean's strength (53 strength has a modifier of +21 - you take 10 away from the stat value and then halve it, rounded down) - so the to-hit modifier is 33 from its base attack bonus, plus 21 from its strength modifier.