Slight changes in Dmax

Hi, I think I may have been in the darkroom too long, but I notices slight variations in exposure between different prints at the same exposure. One print had great dmax and the right exposure, but I could not replicate. I replaced my dev and even the fixer.

I was using split contrast method, f16 20 secs grade 0 and 25 grade 5, with a 5 sec dodge on the grade 5 exposure. I was doing the dodge at different times during the exposure, sometimes leaving the enlarger on for 25 secs, and sometimes stopping after 20 secs and turning enlarger back on for 5 sec dodge. Any of this likely to make a difference?

Your second paragraph already states a variation X 1 zillion.

What kind of light source?