Think of #doggy as a filter on the wider porn caption caption archive: same submission pool, but slimmed down to the exact register you searched for.
Recent uploads land at the top of the vertical feed; older high-score posts anchor the trending row further down; co-occurring tags branch off toward adjacent themes.
Because #doggy is tag-locked, the recommendations under each post also stay in the same lane, which means fewer 'that wasn't what I wanted' clicks and more time on the actual fantasy.
Every visit reshuffles the sort weights slightly, so the same #doggy archive rewards repeat trips — the top of the grid tomorrow won't be identical to today.