There can be large differences between offers for a myriad of reasons and most of them will be legitimate. That is the reason to do just what you are doing, shopping around. There is also bait and switch out there as well as low balling, especially to new travelers.
This is a very competitive business but quote details can be very different. That doesn't necessarily mean the total value is that different. The quality of supplied housing can be very different for example. I really don't know the first thing about the NYC housing market, and very little about the assignments there, but I can easily picture a wide range of housing - it even could be shared. You need to collect all the details to make the best decision for you. Using the calculator found on PanTravelers will put those very different quotes on a level playing field and you will be able to see who is actually paying best when you factor in housing, insurance, licensure reimbursement, and travel.