Can a RAM module not be compatible with Macs?
Since Apple is using the same CPU and mostly intel logic boards, as long as you get the CAS latency and other details exactly perfect, it's hard to imagine you would buy the wrong memory.

Perhaps it boils down to support and testing. If the vendor literally spends more money testing and supporting the Mac memory, it's their privilege to sell it for more. If they don't and are just looking to maximize profit, that happens too. Look at Amazon charging different price based on who they think you are, what you have bought, etc... Same with airlines that charge different prices based on cookies set from previous flight searches.

In the end - all that matters is you understand the return policy and how to test things once you get the RAM. Then it's just a calculation, you estimate how much is your time is worth and you minimize your transaction cost - pay more for an easy buy that saves you time or spend more time looking for and researching a bargain. Perhaps the seller has decided Mac users value their time more highly and you can score a deal by saying you are buying PC RAM.

Happy RAM hunting.