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.
It might be different depending on what type of app you are using, but generally speaking, it sure is beneficial to update from 4 GB to 8 GB since 4 GB is not a lot for a Mac and you are using Xcode.
The iPhoto app might not change much, because it has to load lots of photos, its latency depends more on the disk instead of the memory.
The value of virtual memory might not be very informative, you should look more at swap used right below virtual memory, this is the memory that is actually stored on the disk since the physical memory is not enough.
If you see that swap used is often more than zero, your priority should be absolutely to change for 8 GB memory. Otherwise, installing a SSD might be a better choice though that's more expensive...
edit 1:
Generally speaking, I would recommend upgrading the memory over upgrading to a SSD.
A SSD affects only the loading speed of things, that is, the time that it takes for the computer starts up or for an app to load itself. But the size of memory affects the speed of apps when you are opening lots of them.
On the Mac, the concept is switching more and more to holding the app inactive in the background instead of closing them completely, that enables the apps to respond quickly since we don't have to load them again from disk.
Despite Apple's memory compression technology, opening lots of apps still takes a considerable amount of memory, so upgrading the memory prevents the mac to swap some memory to disk, and makes the mac faster when you have lots of apps in the background.
Best Answer
Yes, there's a problem:
PC3L ≠ PC3.
PC3L is PC3 Low Voltage memory that operates at 1.35V and PC3 operates at 1.5V. The 2012 MacBook Pro calls for PC3-12800. See Everymac.com. For best results you want to use the exact memory, or higher quality memory that operates at both voltage levels.
Also, that Mac is capable of 16GB of RAM, and the price point is low enough that you could max it out for only a marginal amount more. Nobody ever said "Gee I have too much memory!" It's better to have and not need than need and not have!