Hmmm. That’s curious. I can’t say I know enough about chlorine/chloramine to know what could create that difference but it sounds plausible.
It seems like your baseline is about 400ppms. So I’d just approach the nutrient schedule assuming 400 = 0 and add as needed from there. Means for a young plant you’ll want 800-1000ppms which sounds insane but you won’t ever know what’s in the tap without a water quality report.