The developer makes them up.
Minimum = worst computer the devs had kicking around that they tried running the game on and the game ran
Recommended = worst computer the devs had kicking around that they tried running the game on and the game ran pretty well
I'm in publishing and we get told that single-threaded games require i5s, games that never go over 1GB in Task Manager require 8GB systems, and 2D platformers require enthusiast-tier Fermi as bare minimums.
If they use the Additional Notes in sysreqs to say things like "the above config will run 1280x720 at 30FPS" for a minimum and "the above will run 1920x1080 at 60FPS" for a recommended, you can be pretty sure they've put some thought and testing into it. Otherwise the whole section is almost meaningless.