They may be doing this for the specific purpose of attracting attention. Or to give everyone a chance to get a copy. I meank, they could have one huge sale, everything sell out in fifteen minutes, and then get a thousand complaints from those that missed out. Or they can spread it out over a few days and give more people a chance. Or, they might use it to keep people from buying 100 copies at once.
I think a more realistic answer is that the online site may be getting extra inventory from retail stores. So, they'll get a shipment one day of 100+ copies from stores all around the country. Then get another shiopment the next day and so on. Again, it could also be their supplier. Sending shipments that get sold out as fast as the supplier delivers them.