When you got a Survey with for example 16 questions. The first one is about the grade you had in maths in the primary school. Depending on that choice 3 different questions are posted. So the questions 2 to 16 are conditional, due to the grade selected in the first question and each user has only 4 question the get through the survey.

Unfortunately the progress bar depends on the whole amount of questions in the survey. So after question 1 the User only has 6,25% and NOT 25% (what would be right in that case).

That leads to minimize the motivation of the survey users, because they think they're still at the beginning but in real they've nearly finished.