The derating factor is in line with table 4C2. This table only goes up to six circuits (with a maximum derating factor of 0.55 in your case). In the tool for all conditions, we allow up to 20 circuits for all installation conditions. For any number of circuits above 6, will use the maximum derating factor, which is for 6 circuits. Hope this makes sense.
Other derating tables do have more than six (with the maximum being 20).
Please also note that we are not supporting this particular tool going forward. It has been superseded by our cable sizing application, myCableEngineering.com. I would suggest you give this a go, as you have more options and it is far easier to try out different scenarios.