Monkey Grip Feat not giving the -2 to-hit penalty with spiked chains

[Stone Dog]

With Monkey Grip, wearing a spiked chain and a shield doesn't give any penalty to hit.

[Frumple]

This may be because spiked chains technically aren't large (i.e., normally two-handed on normal sized creatures) weapons. They're treated as one size up for the purpose of deciding if it needs two hands, but not anything else related to that. There's few weapons with the same issue -- definitely the double ones (axe, flail), quarterstaves, probably some others.