Ring Gap?

An interesting question indeed. A quick look at the material says that is not likely the difference: thermal conductivities of hypereutectic AL in one study are as high or higher than 2618 AL for all hyper mixes. Conversely, this product sheet shows it to be 13% lower than 2618. So not a huge factor either way. AMC 4632 - Hypereutectic, high tensile modulus aluminum silicon alloy

I would suggest looking at the heat path from the crown down into the rest of the piston. The KB pix above shows the large gap between the top and 2nd ring, and the large round groove between them. The crown thickness in the KB's is not all that thick... maybe in the range of .225-.250" (as opposed to older style cast pistons as an example, where it is commonly .300" thick or more). So with that thin crown, the longer path down into the skirt, and that extra round groove helping to restrict the heat flow path, it may be that the crown and top groove on the KB's just runs hotter due to a poorer heat conduction path. That would put more heat in the top ring.