It's because Black rust is unstable, and subject to oxidation, turning to the more stable Brown form. Once original lacquer is rubbed off over time, this occurs. The lacquer would have to have been periodically renewed to prevent this. Very rare in the vast majority of cases. Even on guns with remaining lacquer on the Action rarely have any remaining on the tubes. On unprotected barrel segments hanging in my shop, they turn noticeably browner after only a matter of months.

Regards
Ken

I prefer wood to plastic, leather to nylon, waxed cotton to Gore-Tex, and split bamboo to graphite.