Question on differential fluid condition

[H8Rain]

I bought my 2005 4.4 new. I have changed the differential fluid (front and rear) every 30k miles. I just hit 120k miles so I changed it as usual.

Every time (all 4 times) I drain the old fluid the rear diff fluid still has a golden color. But the front fluid is dark brown.

Does anyone have any idea why the fluid in the rear stays golden while the front turns dark brown? Just seams strange to me. FYI I always use Redline 75w90

PS: for the record at these 30k diff changes I also do the trans (drain, new filter and fill) and the transfer case. Both with BMW fluids from the dealer.

Thanks!