There is a long-standing debate why the Mn i 5394.7 Å line in the solar irradiance spectrum brightens more at higher activity than other photospheric lines. The claim that this is caused by spectral interlocking to chromospheric emission in the Mg ii h & k lines is disputed. In this paper we settle this issue, using classical one-dimensional modeling for demonstration and modern three-dimensional MHD simulation for verification and analysis. The unusual sensitivity of the Mn i 5394.7 Å line to solar activity is due to its excessive hyperfine structure. This overrides the thermal and granular Doppler smearing through which the other, narrower, photospheric lines lose such sensitivity. We take the nearby Fe i 5395.2 Å line as example of the latter, and analyze the formation of both lines in detail to demonstrate and explain the granular Doppler brightening which affects all narrow photospheric lines. Neither the chromosphere nor Mg ii h & k emission play a role, nor is it correct to describe the activity sensitivity of Mn i 5394.7 Å in terms of plage models with outward increasing temperature contrast. The Mn i 5394.7 Å line represents a proxy diagnostic of strong-field magnetic concentrations in the deep solar photosphere comparable to the G band and the blue wing of Hα, but not a better one than these. The Mn i lines are more promising as diagnostics of weak fields in high-resolution Stokes polarimetry.