What should a load cell response curve look like?
In general, a load cell is designed to have a linear relationship between output voltage and the load applied. In this case it wasn't.

There is a specification missing from the list, the linearity spec. Typically, load cells will specify the deviation from a straight line as a percent of full scale. For example +- 0.2% FS. All that is specified is the Full scale (or endpoint) value which says nothing about the linearity. 

The load cell's output is monotonic, so it is usable. Depending  on your products price and accuracy requirements, this load cell can be used and linearized in firmware by look up tables or a couple of piecewise best fit equations. Alternatively, use a higher quality load cell.