I imagine the poster meant that Alamo keeps prices relatively high until about 2-3 weeks out, and then if they have fewer reservations than they were hoping for, drops prices dramatically in the final couple of weeks.

Rental car companies can shift their inventories from one part of the country to another much more readily than hotels can. And I would think that rental companies rotate through their inventories (acquiring new cars and retiring old cars) pretty rapidly compared to individual hotels (which have fixed inventories of rooms), so they can try to adjust to the economic times by cutting back on their acquisitions and letting their average car age rise.