I don't know exactly what to make of this except that this seems to be a function of the drug test itself
Youre right in your deduction. It is due to the function of the kit. The kits utilize enzyme-based reagents for the reactionary process and the amount of enzymes do vary from kit to kit. Thats the main reason why one kit can produce a lighter (or bolder) line than another kit using the same aliquot.

And, by using the same aliquot, it also reinforces the idea that line intensity is not proportionate to the amount of THCA that is present in the urine, despite popular belief.