I'm confused by the Maximize Contribution to Individual 401K calculation. I was a contractor all year, and have a solo 401K plan. How does this calculation work?

The calculation is done using the Deduction Worksheet for Self-Employed in Chapter 5 of IRA Pub 560 and  TurboTax faithfully implements this calculation.  Because of several different limitations described in the tax code that must all be met simultaneously, the result depends in a complex way on net profit from self employment, your age and amount amount of W-2 income on which you pay Social Security taxes.  Those with income in a certain rage where self-employment net profit minus the deductible portion of self-employment taxes is only a little bit above the regular elective deferral often overlook the limitation that permits the employer contribution to be no more than one-half of the amount that remains after subtracting electives deferrals.

https://www.irs.gov/pub/irs-pdf/p560.pdf