I think it is human nature to under-estimate the work for a project where one is not familiar with the field (or even if one is). I've worked with quite a number of PhD students. The only one who came in under 3 years was the one doing their second PhD, having effectively finished one and moved on to the next. I have seen a few finish in 3.5 years, with an APA scholarship (3 years) plus a university top up (6 months, only for writing).

On the other hand, it also comes around to having a supervisor who knows how to set the appropriate sized project.