The diff threshold sets how much pixel change must accumulate before Lemwatch alerts — default 2% is right for most product pages.
The threshold is the % of pixels that must change (after ignore-zone masking) before an alert fires. Default is 2%, which catches copy edits and layout shifts while ignoring anti-alias jitter.
Site → Visual Monitoring → <page> → Sensitivity.
Browsers subtly re-render fonts and anti-aliasing between runs. 0% would page you constantly.