Different due dates but same breeding dates

Different due dates but same breeding dates

I have a number of animals with the same breeding dates but different due dates. This makes it difficult to sort animals for freshening and for closeup pen moves. There are 2 heifers with the same breeding and are 10 days apart in due dates. And, another example would be 2 cows with the same breeding but only 5 days apart in due dates. 

Why are these dates so inconsistent?