"What accounts for the fact that grosses fluctuate for shows that are consistently sold out without discounts? I understand that there is often a discrepancy between the gross and the attendance due to the fact that many shows have steep discounts and/or comps. But what about shows like Harry Potter? It's never on TDF or TKTS, and as far as I know, they don't have any promo codes, and they haven't been changing their ticket prices. So why would their grosses (and for that matter, the average ticket prices) be different every week, when they always sell the exact same number of seats, seemingly at the exact same prices?"
Because different seats sell at different prices. In some cases, a seat will sell at a premium price far in advance. If that hasn't sold a few days before the performance, they'll drop it down to regular full price. Dynamic pricing is also very much in play these days, so ticket prices will fluctuate. Additionally, various shows - even the big hits - have to comp or discount certain seats for trade/media/PR/employee purposes, which would mean lost revenue one week, and then an uptick on the next one.
-There's the muddle in the middle. There's the puddle where the poodle did the piddle."