My PhD was in biophysics and the questions ranged widely outside my dissertation (in my case, you actually do a defense to proceed to work on your PhD, then write your dissertation and give a final talk; there's no way, unless your advisor rejects your dissertation, that your phd wouldn't be awarded. Other programs rear-load the process and have a real "defense" at the end, which is crazy if you think about it.

At one point in my thesis defense, I derived several equations I hadn't seen before, on the fly, such as "What is the time resolved fluorescence of a fluorophore in 4-dimensional space?" and finally understanding ergodicity (https://en.wikipedia.org/wiki/Ergodicity).

My defense wasn't about determining if I was an expert and qualified to write a dissertation in my field (my questioners already knew that), but to determine if I was a well-rounded general intelligence capable of out-of-task prediction.