I think it's just that Google's instant answer is scraped from endmemo.com, and that website has it wrong.

If you check the second search result (https://www.llojibwe.org/drm/environmental/content/concentra...), it also corroborates what I'm saying:

> 1 nanogram/liter (ng/l) = 1 ppt

Technically speaking, ppt/ppb/ppm should be used for dimensionless quantities, but the convention used here is kinda contrary to that. The argument for the convention is something like this: 1 L ~= 1kg of water, so 10^-12kg of something per 1 L of water can be considered as dimensionless, since the kg and the L cancel out.