A 2-bathroom home in MI, with a water softener.

Our master bath toilet, about 2-3 weeks after being brushed and scrubbed, starts to accumulate and build-up this nasty-looking brown residue in the bowl, like a toilet that hasn't seen a brush in months.

However, the other bathroom toilet doesn't accumulate this nasty-looking brown film, and rarely, if ever, needs brushing.

Does anyone know what causes this brownish film to accumulate, especially in only one toilet, when both receive the same water supply?