A conduit is surcharged (HGL above the top), but the reported depth is shown as being equal to the pipe rise (diameter). Why isn't the depth shown as being equal to the hydraulic grade line minus the physical elevation?
The calculated pipe depth is being "capped" at the pipe rise.
It is not physically possible for the water depth in a pipe to be greater than the rise (diameter), hence the depth is "capped" at the crown. If the conduit is flowing under pressure (surcharged), then the hydraulic grade will be above the crown elevation, but the "depth" result field represents the depth of water in the pipe. Even when the pipe is surcharged, the water is inside the pipe and hence the depth is equal to the rise.
If you're looking to assess the extent to which a pipe is surcharged, for example to assess if a pipe is only 10% surcharged vs 50% surcharged, you could create a formula user data extension that subtracts the start invert from the "hydraulic grade (maximum)" result and expresses it as a percent. Related article:
Creating Formula-based User Data Extensions