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.
All ram modules are non Apple, as apple does not make ram modules.
There are three types of ram modules.
Apple branded, they come in new macs and you buy them from apple at a higher price, guaranteed to work (sometimes they mismatch the type and don't work :)
Other manufacturer branded "mac-compatible"... they are tested by the manufacturer (OWC, Crucial, Corsair, Kingston etc...) and work in macs (sometimes they work in some macs and don't in others)
Other manufacturer branded... untested in macs... MOST work, some don't. Apple uses very tight specs... so some memory does not pass the tests.
How to choose which one. Select 1. if you have money and no time to bother and look around. Select 2. if you don't have that much money but you have a little time to spare and read the manufacturer's spec. Select 3. if you actually read forums and search around for what other people like you bought and tested and it works. Cheapest option.
Conclusion: If you have money you probably have no time to read about "sh*t" like mac memory compatibility.