That's because seats are held to accomodate subscribers and/or donors. The vast majority of exchanges for any given performance happen day of, resulting in open seats- and the box office puts aside seats to accomodate people who are switching in. You'll see this stop happening as a show approaches the end of its run.
"...everyone finally shut up, and the audience could enjoy the beginning of the Anatevka Pogram in peace."
FINDING NEVERLAND needs more than 1 million a week to be called "critic-proof" or on it's way to re-couping. Remember a lot of their current income is from advance sales before the horrid reviews came out. I walked by TKTS and NEVERLAND was on the board and a scalper was hawking NEVERLAND tickets also. (as I passed him, he mumbled under his breath, "nobody wants to see this turkey"). Only the future will tell, ask again in the Fall.
"FINDING NEVERLAND needs more than 1 million a week to be called "critic-proof" or on it's way to re-couping."
...it HAS grossed over a million per week since their second week of previews. The thing is no where near recouping but - so far, so good. Whether or not that continues remains to be seen, but you can't deny those numbers.