(1) Maybe one is older than the other and breaking down..?

(2) Maybe they are of different composition (the 10.000 being an earlier production)..?

I know of one case for sure where a casino ordered 100 pieces of one denomination in 1964 and in 1976 reordered an additional 100 pieces of exactly the same size, shape and color... and then followed up with another denomination reorder three years later -- with all pieces in the series matching; exactly, and there wasn't any noticed difference... but that was in 1996.

(3) Perhaps age is taking its toll on the 10.000 you show..?

(4) What is the difference in the 10.000 and 50.000 you show (odor)...?