The 3rd does mean something, it's a further breakdown, hence the 775, 765, 755, etc. exist.
I looked up game debate (which is useless btw with no real world comparison http://www.game-debate.com/gpu/index.php?gid=2206&gid2=1247&compare=geforce-gtx-850m-4gb-ddr3-vs-geforce-gt-740m) and benchmarks and they show the 850m is nearly double the performance of a 740m. The why is simple, just look at those specs.
http://www.notebookcheck.net/NVIDIA-GeForce-GTX-850M.107795.0.html
http://www.notebookcheck.net/NVIDIA-GeForce-GT-740M.89900.0.html