Contractors can overcharge you for Materials or simply buy more than they need and keep them for the next job or return them for a refund.

Don't rely on the one quote from the GC as many are over-charging for repairs.

Add that to the fact that rents have been rising faster than inflation these past few years and the result is investors getting over charged for PM services.I have seen some PM companies already adopt new pricing structures which actually reflect the amount of work done.  

My property manager has been severely overcharging me for repairs for months. 

Handyman will definitely over charge you and bill for things not done.

Large companies billed incorrectly and overcharged for years. 

The housing board sided with me and we actually had our rent reduced because she found that the landlord was overcharging us rent for 2 years running.  

Account Closed you need to verify that what they charge you is actually been repairs and done. there are many horror stories about PM's that over charge. 

I also worked with Empire Property Management, their sister company, which ripped us off on the PM aspect of the business by frequently and overcharging us on many maintenance requests (they have their in-house contractors).

Maybe you are being over charged for fixing things or they aren’t putting in quality tenants.