Minimum Timing Differences??

[1BADVAN]

I have been playing around a bunch with the timing on MPTune, and i noticed something that in my cal which started as a T-SMEC 2.5 ATX cal will have the "calculated timing" 3D table go to a minimum of "0" but if i use the base call of the Boostbutton Stage 4 cal no matter what i do the timing never goes below 15.5 on the 3D table.

Why is that?