This is because a constituent cannot have the same address twice, including the same zip code, if those addresses have the same type.

For example, a constituent could have both the addresses below

Home Address 
123 Main Street
Exampletown, CA 90210-1234

and 

Home Address 
123 Main Street
Exampletown, CA 90210

OR 

Home Address 
123 Main Street
Exampletown, CA 90210-1234

and 

Seasonal Address 
123 Main Street
Exampletown, CA 90210-1234

But a constituent could not have both of the addresses below:

Home Address 
123 Main Street
Exampletown, CA 90210-1234

and 

Home Address 
123 Main Street
Exampletown, CA 90210-1234