AutoCAD Civil 3D - Stormwater

[#1]

I set up model for two options for a same drainage area of 38.8 acres.

Option 1: Gutter flow upstream and piping flow downstream

************************** Volume Volume
Flow Routing Continuity acre-ft Mgallons
************************** --------- ---------
External Inflow .......... 0.000       0.000
External Outflow ......... 7.733     2.520
Initial Stored Volume .... 0.000    0.000
Final Stored Volume ...... 1.284   0.419
Continuity Error (%) ..... 0.133

Option 2: Piping flow for the entire area

************************** Volume Volume
Flow Routing Continuity acre-ft Mgallons
************************** --------- ---------
External Inflow .......... 0.000 0.000
External Outflow ......... 14.791 4.820
Initial Stored Volume .... 0.000 0.000
Final Stored Volume ...... 0.127 0.041
Continuity Error (%) ..... 0.008

My problem is: Why is the difference of external outflow volume so significant, 7.733 and 14.791? Shouldn't the two values be close, if not exactly the same? Did I set up something wrong in Option 1 which includes gutter flow? Pleseae see the attachments of two options for reference. Thanks a lot.

[#2]

Option 1 allows flows to travel along the curb and gutter system (which possibly has different properties from the pipe network and in some case increases TOC) prior to it entering the pipe network below. Thus, longer TOC for the time span would reduce outflow volume. Option 2 allows all flows to enter directly into the system without having to travel through a curb and gutter system first. This results in a single TOC which would be based on the parameters of your pipe network. If the two time spans are identical in both options it is possible that option 1 show less volume than option 2.

[#3]

I can understand the Peak Flow would be much lower for gutter flow because of TOC. But for the same storm event in a same area, the outflow voulme shouldn't be the same?