What would cause a properly exposed film to appear dark?

The film appearing dark can be attributed to overdevelopment, where the film is left in the developing solution for too long, resulting in excess blackness as more of the silver halide crystals are reduced. This usually occurs when development times, temperatures, or chemical concentrations are improperly managed.

Underdevelopment would generally lead to a film appearing too light, as it would not have had enough time to develop the necessary contrast. Incorrect exposure can result in various outcomes depending on whether it was underexposed (too light) or overexposed (too dark). However, in the context of dark film specifically, overdevelopment is the primary cause.

Considering both overdevelopment and underdevelopment provides a broader understanding of how film processing influences the final appearance of radiographs. Overdevelopment indeed stands out as the leading cause for why a properly exposed film might appear excessively dark.