Here's a link to the trading statement....
Alamy stock photography - Trading statement
Alamy is a non-exclusive agency. NOTHING needs to be exclusive. The only time it may be a good idea is if you are selling under a rights protected license. Essentially, if a buyer wants exclusive rights to an image in North America, you need to be able to restrict that image at your other traditional agencies. You can still license the image in other areas of the world. Selling RP allows them to say yes without contacting you about the arrangement.
The same can happen with RM - however Alamy will contact you first, before they promise exclusive rights to the customer.
The reason RM sells more on Alamy is because they sell a lot of editorial images which are RM in nature (a lot of it being editorial travel related images).