Δ since prev is the real gap between this press and the previous one, across any button. SAME POLL means two presses landed in the same Gamepad API read (~the browser's polling resolution) — closer than that can't be distinguished. Latency is time from the real controller's hardware timestamp (or the raw keyboard/click event, in Virtual mode) to when Padlab detected it.
Leave sticks untouched, then start calibration to measure drift.