Moulin Rouge has been doing this too. If you look at a random date, months from now, you are able to see which seats are clearly being held (usually the first 4 aisle seats in the right and left orch for the first 8 rows or so, as well as a number of center seats) as they are unavailable every performance. Meanwhile - they release them about 2 days prior to the show - like you said - usually at a better price.