question re: emerald sizes

[Cravin My Emerald cut]

I was using this chart as a guide and have a question that I''m hoping someone can answer.

http://images.amazon.com/media/i3d/01/actual-diamond-size.pdf

Why is it that for most 0.5 ct increments the dimensions increase 1mm in each direction EXCEPT for the increase from 1.5-2 and 2-2.5?

Thanks!

 

[chrono]

As the carat weight goes up, the increase in increment decreases. More weight will always be in the pavilion, so the increase will never be linear.

 

[Cravin My Emerald cut]

I can understand that...I just found it strange that between 0.5 and 5 carats it ONLY happened at those weights.