Why Scaled length is measured in different ways in the Subsurface Utilities product & H&H products.
When the hydraulic calculations are considered the length is measured from center to center, whereas when you are measuring the length with Subsurface Utilities, the length measured is face to face length.
Here is the look at the length measured for hydraulic calculations and using Subsurface Utilities; i.e., lengths measured center to center and face to face.
Even if you are importing the project between Subsurface Utilities and OpenFlows products, the length remains same making no difference in the hydraulic lengths. (See the screenshot below). It shows that the center-to-center length is considered for hydraulic calculations and the Subsurface Utilities product as well. This means even if you are moving the project between these products, there will be no change in the results as center to center length is considered in both the products.
Whereas face-to-face length measured in Subsurface Utilities is considered while building the cells and 3D models.
Note that you may also see small differences in the center-to-center length due to the rounding option available in the OpenFlows product. In Tools > Options > Project you will see an option to "Round pipe lengths to the nearest:", which will round the length values used in the profiles. There is also a check box to calculate pipe lengths using node elevations. This is referred to as "3D" length, because it accounts for the difference in elevation between end nodes, as opposed to a bird's eye view.
Center-to-center vs wall-to-wall length and slope calculations