I think university faculty earn much more than poly lecturers, and rightly so, because the bar for entry is much higher. For one, you would need to have a PhD to be placed on the professorial track. This is after slogging for a few years on a stipend while your peers are getting increments and bonuses. And getting hired does not guarantee stability. Academia is very competitive now. As an assistant professor, you are on contract, and if you do not perform within a certain time frame, you have to leave.