Pickles,
Your question made me think for a while. Usually I am trying to avoid using a constant spring.
There is another parameter that causes a selection to be a constant spring hanger. If the available space for spring hanger is not sufficient, the selection defaults to the constant hanger. For example, if the Anvil Fig. 82 height dimension is input for the space available, but the spring selection requires a Fig. B-268, or even a Fig. 98 with double stack spring, then the selection defaults to a constant spring hanger.
I think if you input a small available space, say 4 inches, then the constant spring selection will be the result for any movement greater than the rigid support displacement criteria.