A lot of valuation come from how valuable the owner is to the day to day operations of the business. If a business is systematized properly, the business has way more value, because an experienced investor can easily come in and operate the business. If the technical expertise is all invested in the seller, it makes it a lot more difficult to transfer over to a new owner.
Also, if you are buying blue sky, or reputation, it is a lot harder to sell and has a lower value than solid, long term contracts. Face it, you could have a residential business that services 100,000 homes a year, but none of them will be under contract, and so, even though the original owner might see residual effects and service calls, in reality, they are all blue sky because there is so little guaranteed, even if it is psychologically.
Thus the hazards of residential and non recurring customers.