"Shouldn't they mutually exclude each other?"

I can't speak for the mfgrs intention but I believe the two are redundant safty nets for one another. In your case I'll bet the filter wasn't restricted to the point pressure monitor would activate warning. As anyone who changes a lot of filters in various households can tell you,there is a wide descripancy in filter life from home to home.

Thank you for the reply. The filter was rated "Up to 12 months", so I guess I can I keep it there for a few more months.

"The filter was rated "Up to 12 months",

Hee hee,don't count on it,the key is UP TO. Rely on routine inspections and your pressure monitor to decide when it's time to change. If a filter is dirty after only 1 month of use it's time to replace it. If a filter go's 6 months and still isn't dirty, it's wastful to change at the time. As usual,there can be exceptions,like in 6 months without becoming restricted by dirt ,prudent to make certain air isn't bypassing filter.