Can someone tell me why my rMBP Early 2013 15" with 650M will drive a Thunderbolt Display 2k @ 60hz but wont drive 60hz on an HP z27s at 2k?

I've figured out that it will but had to download SwitchRes X to do it. It sacrifices the HDPI though. Not sure why because the thunderbolt display is def HDPI. Getting it to 60hz is a must for video playback/editing. Don't really see much degradation in the video with out the HDPI but would still be better to have the ability. Still don't get why the 650M will drive a thunderbolt display at 2560x1440 @60hz with HDPI but not with an HP z27s at that same setting. Maybe the thunderbolt display has an internal refresh "amp"? I don't know if that makes any sense. Maybe it has something to do with internal makeup of pixel science between the displays, I don't know. I am going to try a mini dp to mini dp cable that states it'll do 60hz, but we'll see.

The information out there when buying this monitor was vastly varied and hard to find. Even the specs for this macbook pro version was essentially incorrect on apple.com. This machine will drive higher that 2560x1440 natively (3200X1800) but only at 30hz. When looking into the HPZ27s I was ok with the fact of only running 2k but had to be 60hz, just like my thunderbolt display. I went with it knowing that it'll be great for when I get a chance to upgrade my machine.