Is there a problem with GeForce 750M performance on gaming?

I play games on my Mid-2014 retina MBP, which has both Iris Pro and GeForce 750M 2GB. One of the games that I play (League of Legends, which has a 'beta' OS X client, but I highly doubt that this is the reason) sometimes runs slow on graphics-intensive scenes. Today, I forgot my GPU on 'Integrated only' (I use gfxCardStatus) and entered the game. Interestingly, the game almost never slowed down at all, even on graphics intensive scenes, running smoothly on 60FPS. I've upped the settings and it was still running smoothly at settings which severely affected GT750M's performance. I've doubled checked the active GPU and yes it is Intel Iris Pro. How can this happen? How can integrated graphics perform better than a GT750M on gaming? I know Iris Pro is a great 'integrated' card (compared to other integrated cards), but I highly doubt that it does better than 750M. If it really did, then why would it co-exist with 750M anyway?

Why would this be happening?