Screenshots taken from MtGox homepage - click on "charts" section, a thin ribbon at the top with current valuations.
Is this caused by rounding? What if the spread was less than one cent?
On 920 trades? This doesn't sound right. And their JSON data isn't consistent with that.
Honestly, it sounds like a display bug. I think that graph is intended for casual observers not real traders considering how low-res it is. As long as the tape is correct, who cares.