Digital BW, The Print

I've just picked up a PrintFix Pro and have been using it to build a warm
curve for Innova Soft Texture Art. I'm using an Epson 2200 and UC inks.

My Dmax readings have been 1.61 or 1.62. All of a sudden I'm getting 1.56,
1.57 and/or 1.58. A curve that measured 1.61 when reprinted (exact same
settings) now measures 1.57. I've remeasured the original curve and it's
still 1.61 (spectro is consistent). The matte black ink has not been
changed.

Any idea what might have happened? Is the difference worth worrying about
(am I splitting hairs)?

Thanks,

Terry.