Trouble with Power supply

Hey so I am following the write up for the $100 laser set up and I am currently at the point where I am testing my TTL power output after switching the fan output from D9 to Pin 44. When I put the fan to max 255 and test it while its hooked up to my computer and I am reading 4.9 volts but when I connect it to my power supply 12v 30Amp regulated power supply and check the reading it for TTL it is higher its around 6.9V. does anyone have any insight on why the power supply vs PC are getting different readings?