Hi @jacksta. We had a similar scenario with quotes for the first of our two systems. Fortunately the issue was evident before selecting the best supplier from the quotes. I was able to get amended pricing before making the decision and assess the differences.
Unless for the quote there has been someone on your roof to measure up it is open to question if they are being optimistic to maximise your outcomes or just lazy. They all seem to rely on using a high resolution aerial image and guess of the roof pitch. Reducing the cost by $500 for one less panel seems very fair/generous.
The installing team is typically not involved with the quote. For our system the installer turned up with a full set of 24 panels when the quote and design was for 18. We installed 20. The quoted design layout was not what the installer recommended should be done. It wasted 30 minutes at the start of the day to sort out. I’d suggest it is less deliberate and more just poor management.
STC’s are based on the installed panel capacity. It’s getting close to two years since our systems were quoted and installed. I recollect receiving copies of the paperwork submitted by the supplier/installer with the STC’s and final system approvals, (not the quoted details). Hopefully these match your 21 panel install.
Changing how solar PV panels are arranged on a roof changes the amount of effort and materials (rails, brackets etc) required for the install. Subcontracts or human nature considered the best layout of panels for solar generation may not be the easiest or cheapest for the installer. Householder be well informed and have the design in black and white. Although we received quotes stating the panel arrangement was only indicative. IE subject to change on the day. The latter makes a joke of CEC accredited suppliers required to have an accredited solar system designer.