I find a similar effect. It looks to me like most people systematically overstate probabilistic claims above their overestimation of certainty.

So that when they say P(?) = C, their internal estimate of P(?) = C(1-delta), while the long run expectation when they say P(?) = C is more like E(?) = C(1-delta)(1-gamma).

So when you say it, they downgrade what you say by (1-delta).

Kind of a Gresham's law for probabilistic predictions - over confident predictions drive out appropriately confident predictions.