A simpler reason is usually that a lot of people know that others are prog lying, and decide to go for a higher prog point to make up for this, consciously or unconsciously thinking that if they go for prog point +1, then people that will join are actually only at prog point 0. It results into a snowballing effect, a bit like when you have someone early pulling by 1s each pull and the tanks get annoyed and start pulling at -1 or -2, and the player in turn starts pulling at -3, etc.