AutoCAD Civil 3D :: SSA Rational Method

SSA rational method. I have to compare results of rational method between SSA and another product. Plain rational method, steady flow, Darcy-Weisbach. Times of concentration (TOC) are manually defined.
 
I played a bit with the units, formulas, IDF curve is defined as pair of values time/intensity. Seems that I achieved something. Results are more or less similar, but there are some differences. After playing with both programs, comparing in Excel, think that I found what is different.
 
So, I have very simple example, of one channel with catchment areas. All areas have the same runoff coefficient – 0.8. Pipe diameters are constant (500). The whole analysis is restricted between 5 and 10 minutes of rain duration (5 min-397.22 l/s/ha and 10 min – 321.53 l/s/ha). I converted IDF to mm/hr. So I have the next example:
 
[picture in attachment]
 
In theory, what I learned about rational method, there is next definition:

Intensity from IDF curve is defined by TOC. TOC is defined by sum of „entrance time“ (which is defined by shape and size of catchment area) and sum of upstream flow times. In my example I defined 5 min for every area.

So, for example for pipe nr. 3, TOC is calculated:
 
TOC_3 = Entrance time for area 3 + Tflow_1 + Tflow_2 + Tflow_3
 
For example:
 
TOC_3 = 5 min + 0.67 + 0.45 + 0.51 = 6.63 min
 
That TOC_3 (6.63 min) is base for reading appropriate value of intensity (I_3) from IDF curve. The flow in pipe 3 is then calculated:
 
Q_3 = I_3 * (A1 * k1 + A2 * k2 + A3 * k3)
 
This is the way how I learned rational method. But results between SSA and another product (which works in a way explained above) are different. Differences increase downstream.
 
After playing with results in Excel, I found out that SSA does not summarize in TOC all flow times upstream. So TOC in SSA is calculated simply by initial entrance time and flow time in specific pipe:
 
TOC_3 = Entrance time for section 3 + T_flow_3
 
So, there is no time added from T_flow of pipes 1 and 2. Seems that it is major difference which leads to final results differences. I tried to understand all options of SSA to find possibility to add upstream flow times, but without success.