I think you may be confusing yourself with your test. I'm not familiar with that exact camera, but almost all SLRs use a "traveling slit" shutter at the higher speeds. In other words, before the leading shutter curtain has gotten completely across the gap, the trailing curtain is following behind it. The entire shutter is never open simultaneously. (This is the very definition of the "speeds higher than flash sync speeds" as you can't get a full image in the split second of the flash firing, if the shutter isn't completely open at the time.)

I bet if you take pictures with it they will come out fine.