I contributed to my HSA pre-taxed through my employer (W-2, box 12, W). When I completed the HSA Summary section, why did the amount of my refund increase?

Until you complete the HSA section to indicate your eligibility to contribute to an HSA, TurboTax treats the HSA contribution made through your employer as an excess contribution subject to a 6% excess contribution penalty.  Once you completed that section, TurboTax removed the penalty, resulting in the increase in your refund.

That makes perfect sense.  Thanks dmertz for solving another tax question!