Phonak Target: changing fitting formula changes estimated feedback threshold?

Target 9.0; no HA ever connected for current client. For purposes of this observation, there are two groups of fitting formulas:
Group 1: DSL v5a Adult; DSL v5a Pediatric.
Group 2: DSL [i/o]; the three NAL formulas; the two ADP formulas.

For a given coupling specification, changing from one formula in a group to another in the same group doesn’t change the estimated feedback threshold dashed red lines. But changing from a formula in one group to any in another group does substantially change the feedback threshold curves.

The same applies to the gray maximum gain curves.

Why would changing the fitting formula change the estimated feedback threshold or maximum gain – aren’t these estimates both entirely a function of hardware (HA and coupling) when no user/client response has ever been measured?