Try not to get bogged down with overanalysis when using THC test kits and interpreting their result.
To conclude that the intensity of the line dictates THCA concentration would be speculating, because assays lack the ability to quantify THCA concentration. Assays are a pass/fail endeavor.
Reactionary process from the regents vary from one test kit to another. Thats why a clean sample/synthetic can produce a bold line on one kit, and a faint line an another kit at a given time. The fact that a bold line was produced after fluid consumption is coincidental.

With all that being said, a faint line in the 'Test' region is still a negative. Dont sweat the small stuff.