Experience it yourself:

  1. Invite four teachers in a room to score 20 essays, math problems or lab reports using a 10 point scale. 
  2. Look at the diversity of scores among the 20 essays, math problems or lab reports. 
  3. Now, ask the same four teachers to score the same 20 student work samples using a *4-point scale.
  4. Once again, examine the diversity of scores among the 20 essays, math problems or lab reports. 
*The same exercise could be done using a 3-point scale or a 5-point scale.

--
I (sort of) gave this exercise a try once and concluded "When using a smaller scale (4 scoring possibilities vs. 10 scoring possibilities), mathematical logic kicks in: humans are more consistent when given fewer scoring possibilities."