Why are there double EMH's (morn & evening)?

[NvrBnToWDW]

We're going May 7-14 and I noticed that several days have Morn EMH and Evening EMH at the same park. Why is that?

 

[marriedmarylander]

I'm going May 16-22 and there are morning and evening EMH at MK, EP on two days when I will be there. At first it was just an evening and they added more mornings.

I actually wrote Disney and asked but they didn't answer my question and gave some vague generalization about park crowds.