Yes, I think the questions do get progressively harder, so the producers can make a fair guess that within a certain amount of time, only three will be left standing.
On shows like this, they know that the maximum length of the programme will be (say) 24 minutes, and that's what they budget for. Obviously, sometimes they have a very stupid bunch who don't last very long, at which point William G fills the time by inviting us to apply for the show, or goes off on a rant about the Elgin Marbles...
You used to be able to tell how long part two was going to be (and therefore whether anyone was likely to get a high score) by how they edited the introductions at the start. If they told you all about all three of them - their hobbies, their jobs, etc - you knew they were filling for time, and that no-one was going to survive through all 40 questions. Which rather took the (admittedly limited) excitement out of it.
A nerdish observation perhaps, but true!