It may just be that specific color -- some color formulas (and ingredients) weigh more than others. Some vary dramatically.
And historically, hot-stamped chips weigh more than chips with inlays, given that all other variables remain the same -- hot-stamped clay weighs more vs when it's displaced by lighter inlay materials (paper, linen, or vinyl) during the pressing process.
But Burt Co./ASM has never used lead in any of their chip composition formulas, although a few chips they produced have contained metal weights hidden under the inlays.