Is this fuse time-current curve wrong?
They don't match the blow current. The graphs are the current that the fuses blow at (not the rating with one number). The reason for this being is it takes temperature to blow the fuses and then the fuse material must break down. This process does not happen instantaneously, its a phase change process. So the longer you dwell above the temperature, the more it will break down. If the current is exactly at the point of breakdown, it could take a long time to break down the fuse material down.  

To really blow a fuse, you must go beyond the blow current, the more you go beyond the blow current, the faster it will blow.

It's also why currents close to the blow current should not be approached for long periods of time (like between 700mA and 800mA) as the curve probably also extends to this region but was not tested.