trash (rats' food source) is usually picked up twice a week in most places in the city, it never lines up perfectly to 'half-way through the week', and sometimes the 'other' collection is 'just recycling' (less organic matter to feast on)

I'm guessing the most frequent submitters send consistent reports when they can see rats on their block going after the trash (twice-ish a week); if more people in different areas reported as consistently, it might even out (some places have trash pickup monday, some tuesday, some wednesday...) (assuming all areas have equal concentrations of rat)