Consistency Theory - Why do we go back on promises
There's no reason that consistency and promise-breaking can't both be pervasive.  In the first place, if 20% of the population always broke promises and another 20% always obeyed the consistency principle then we would still call it pervasive and we still have 60% of the population.

But an additional complication is that no person always breaks promises or always obeys consistency and most people do both at some point. We can imagine a scenario in which a politician could simultaneously break a promise (to a rival political force) and obey consistency principle (keeping the promise would have betrayed his base).  So the two aren't really contradictory since consistency principle doesn't require one to be consistent about everything always.