That has to do with the design of the piston, the material specifically. It "reflects" heat a bit better than plain cast pistons so the burn will be a little hotter. Honestly - I'd ignore that...lol. What you cannot have, in any way shape or form, is pinging or detonation. Those pistons are like tool steel: strong, but brittle. Pinging and detonation are like hammering on a chisel. Parts tend to chip off... 4° of timing more than likely will not make or break that piston. Steady detonation eventually would. Make sure the top ring gap is per the KB specs, and don't sweat the timing unless it pings. Given the static ratio and the cam, you might have to play with the timing curves and carb jetting to keep it out of detonation and ping.