Share 'Windows: Different Summer and Winter U values?'

Anyone ever hear of a window, or unit of insulated glass, as having different winter and summer U values?    This wasn't on an NFRC sticker.  It was on a glass manufacturer's spec sheet.    They listed it like this:   Winter - Air / Argon  0.35 / 0.31 Summer - Air / Argon 0.32 / 0.28   Our program requires a U value of 0.30 to receive an incentive rebate.  So obviously, homeowners and contractors…