Define targets to hit for job completion? Finished within hours, like a range. To avoid your problem right now. Consistently coming in very under hours, I think is a problem. Some one is over estimating too much, possibly to pad their bonus like you say.
If a job is quoted at 100 hours, and comes in completed at 90, bonus time! Job quoted at 100 hours, finished in 70? No bonus, there's a problem. Why so many extra hours?
Does the PM's hours get put in the quote? If yes, are they broken out, Installation Hours, PM hours. Are installs coming in right on the money and PM hours low? Or vise versa.
Do they get paid out at the end of every job, or is it all tallied up at the end of the year? Could try doing bonus based on the year instead. Factor in job wins and job losses, as well as how many hours under or over.
I'm mostly spit balling here, I never received bonus when I was acting PM, but it was discussed that if I ever went full time, I may get a bonus (in lieu of a raise, hah). But I can't imagine I would intentionally add extra hours to make sure I get a sweet bonus, but I know some people are like that.