One inbox, read by the person who wrote the step detector:
Undercounting: where was the phone (hand, pocket, bag, treadmill rail), and did the status line say "rhythm locked"? Bags swing; the rhythm gate is skeptical of bags. That's a design choice, but tell us anyway.
Overcounting: genuinely rare with the gate — if you can reproduce it (say, a bumpy bus ride), that's a report we'd love. Device model and browser included, please.
Nothing counts at all: first check it's a phone or tablet on HTTPS, then that motion permission wasn't declined (iPhone: close and reopen the tab to be re-asked). Desktop machines have no accelerometer — the tool says so rather than pretending.
See or recover your sessions — they exist only in your browser's storage (privacy). Export CSV backups if the history matters to you.