The BIW Toothbrushes have approx. 5100 Bristles, and that is because there is a white spot in the middle of the brush to indicate the user where is the black toothpaste.
Reason:
-the black bristles have Diameter 0.10 mm (same as the CS 5460)
-the white bristles have Diameter 0.12mm (same as the CS 3960)
-the white bristles are a little bit stronger, because it is easier to clean the CS 3960 bristles from the black carbon toothpaste.
So, if the white bristles would have a Diameter 0.10mm – the bristles would stay gray = ugly.
You should not speak about it if not asked tho because it is less than 5460.