Also, were you printing the same or a different negative? Unless you have mastered exposure and development, they probably vary in density. Films have different base fog, as well.
Not being sure if you were printing at the same aperture, could easily account for it as well. A 5 second exposure at f4, would be 10 at f5.6, and 20 at f8.
I usually aim for more than a 10 second exposure, it makes burning and dodging easier.
Keep notes!! I write down the paper, the height of the enlarger, the f stop, exposure time, burning and dodging (with a diagram), and notes about development, if anything other than Ilford MG for 2 min, for each print.