The calculated slope value shown in the software is slightly off compared to a hand calculation based on the pipe length and invert difference.
For example, calculating the inverts using a slope of 0.50% across two segments of pipe, the program may show a slope of 0.48% for one segment and 0.52% on the next.
This is likely a rounding issue. Go to Tools > Options > Hydraulic Model or File > Options > Hydraulic Model (for Connect Edition) or Tools > Options > Project, for (for V8i)
Check the field called "round pipe length to nearest". If you change this to 0.0, the pipe lengths will not be rounded and slope calculations should be exactly what is expected. You may need to re-compute the model.
Note: when using scaled lengths, pipe slope is based on the center-to-center distance between the nodes. In other words, the endpoint vault width is currently not considered. See:
Center-to-center vs wall-to-wall length and slope calculations
Scaled Length difference between Civil / Subsurface Utilities products & OpenFlows products
What does the "Unified length" / "Length (Unified)" field represent?