First of all: Congratulations! I hope you had a lot of fun, and that you'll go back to making more prints soon.

Did the print calculator work in 'bursts', so that the light was turned on/off several times, and the exposure time was cumulative?
If so, the light intensity when the light bulb gets energized is not the same as when it's at normal operating temperature; it takes a little while for it to be warmed up to perform as specified.
That could be a contributing factor, and if you do your test strip this way, you should also do your final print the same way.

Another thing that comes to mind: Did you compare completely dry test strips with completely dry work prints? There is the matter of dry-down to worry about, where a print will darken as it dries.

Finally, did you develop the prints the same amount of time exactly?