Here’s a few threads about it. My theory is that it’s purging 0.4mm amounts but it’s just a guess
https://www.reddit.com/r/BambuLab/comments/1ka2nq9/purge_with_02mm_nozzle_on_h2d/
https://www.reddit.com/r/BambuLab/comments/1jwwuqt/purge_issues_with_h2d_02mm_nozzle/