When a company is doing a one time or initial cleaning then there is more detail and time involved. They may pull out the appliances for example and clean the walls and floors where they stood. It all depends on the needs of the customer.  Cleaning out appliances like refrigerators and ovens are very labor intensive. Once that initial cleaning is done though, then the follow up cleanings take less time because their maintaining what they've already deep cleaned at that point. So that results in a decreased cost.
It's hard to say why a person is being charged less than $150 for cleaning a 2000+ sq. ft. home without knowing specifically what their required tasks are and the time it takes them.