TAZ 6 X and Y steps per mm "fudge factor"?

I noticed in the Marlin code that the steps per mm constant is defined based on the printer configuration being compiled. In the file Conditionals_Lulzbot.h exists the following:

#if defined(LULZBOT_USE_Z_SCREW)
    // Older printers had a fudge factor for ABS shrinkage.
    #define LULZBOT_XY_STEPS                      100.5
    // In the Mini 2 and TAZ 7 going forward, use true XY steps.
    #define LULZBOT_XY_STEPS                      100

Why is this “fudge factor” not needed on the non “LULZBOT_USE_Z_SCREW” printers? What relationship exists between a machine with Z lead screws and XY axis and why do the newer machines use true XY steps?