Six single‑core 2.5 mm² cables fit in a 20 mm conduit, at a fill of 34.56 %. AS/NZS 3000:2018 Appendix C Paragraph C6.2 sets the space factor the tool uses: 40 % for this number of cables.
The case
The conduit's internal area is 176.71 mm². The six cables take 61.07 mm² of it. A seventh cable takes the fill to 40.32 %, over the limit, and the tool then recommends 25 mm conduit.
Before you install
This count comes from the fill method. Appendix C Tables C10 to C12 also give a maximum-count guide, which can differ. Use the method your design states.
The fill method uses each cable's overall diameter. A multicore cable, or a cable with a thicker sheath, takes more room than its conductor size suggests. Enter the cables you will pull, and leave room for the draw-in.
See also: AS/NZS 3000 Appendix C conduit space factor and Conduit fill and installation methods.