what interpretation of the word "probability" does allow you to think that the probability of something is 1 and then change to something other than 1?

They need to have inconsistent attitudes about how they calculate probability, or estimate probabilities by inherently irrational means such as assigning likelyhood based on what hypothesis they want to be true the most and acting like that belief is certain. Empirically, I've met individuals who claim that no amount of evidence would alter some of their beliefs so something like this may be going on. It is however possible that trying to model these beliefs as probabilities implies a degree of rationality that they simply lack. The human mind is not generally a good Bayesian.