1/3 increase for toe pressures....why?

I can think of two possibilities:

1) Many eccentrically loaded footings get their eccentricity of load by way of transient loads such as wind and earthquake. ASD load combinations do -- or at least used to -- allow a 1/3 increase for that. But then, as you suggested, this would be dependent on the load source.

2) For shallow foundations governed by settlement, which is to say most of them, it doesn't make sense to limit bearing capacity to the allowable soil stress at the footing toe where the value is at it's peak. Something closer to the average stress under the footing is a better predictor of settlement. That may be what's happening.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.