The numbers on the Wikipedia page are a little messed up, so you can't do apples-to-apples comparisons. You'd need to go to the actual payload planner's guides to see what the performance is to what orbits. Here are the guides:
Delta IV Launch Services User's Guide
Proton Launch System Mission Planner's Guide
A Proton launch to a LEO with the launch site inclination (51.5°) is 23 t. A Delta IVH launch to a LEO with the launch site inclination (28.7°) is 28.79 t. Quite a bit more. The 23 t on the Wikipedia page is for a Delta IVH launch to a polar orbit (90°), at 23.56 t. So you would expect the Delta IVH to also deliver quite a bit more to GTO or TLI.
(You said GEO, but you must mean GTO, since launch vehicles rarely if ever deliver a spacecraft all the way to GEO -- they drop off the spacecraft in GTO, Geosynchronous Transfer Orbit, which has an apogee around GEO and a low perigee).
What's more, the Proton is handicapped with a launch site so far to the North. Large plane change maneuvers are required to reduce the inclination of the orbit to drop off the spacecraft closer to (but still not at) the GEO inclination of 0°. This figure from the guide shows how large the plane change is:
The 6.92 t GTO figure for the Proton is a drop off to a 31.1° inclination orbit, with a perigee at 2175 km. That leaves 1800 m/s to the spacecraft to get to GSO.
The Delta IVH delivers 14.22 t to a 27° orbit, requiring essentially no plane changes to get there, with a 185 km perigee leaving 1804 m/s to the spacecraft to get to GSO.
The Proton is even further handicapped by a lower Isp upper stage (the Бриз-М or Breeze-M) using storable propellants, as compared to the Delta IVH upper stage using LH2/LOX. So there will be lower performance for the Proton as compared to the Delta IVH when boosting past LEO, even if they were at the same starting point in orbit and mass.
