Thanks! The algorithm works iteratively, so essentially each iteration it makes small changes and then checks if these changes are optimal. (There are many more details described on the wiki [1])

The frames of the glasses were preserved because the algorithm "decided" that it was optimal to keep them with the limited number of points it has to work with.

[1] https://github.com/RH12503/Triangula/wiki/Explanation-of-the...