UPDATE: statistics lagged by as much as three days plus recently (September, 2015). Recently, a message apologized for the delay and said that the statistics will be updated soon; and they were the next day. Therefore, if you have a new application, and you are wondering where your statistics are, relax - "coming soon", as they say in Carribbean.
One can imagine what might be causing this. With over a million apps on the Google Play store, sanding all this data probably takes a considerable amount of time.
Previous answer:
My own recent experience, nine months later than this question was posted, is that the statistics are actually being updated on my developer's toolbar a little earlier than 6:00 AM PST and that the updated statistics reflect what was true at some point at midnight or later that day (i.e., between zero and six hours earlier).
I can imagine some kind of process that starts at midnight and is shredded through all hundreds of thousands of applications, and each of them takes a picture and publishes it. So if your application is at the front end of the list for this process, you may receive your update closer to midnight, and if not, maybe not. Mine always arrives a little earlier than 6:00 in the morning.
I would also like to note that the report on how many instances of my applications were installed on the public page for the applications themselves seems to lag behind the statistics available on the developer's toolbar by five days (until now). Perhaps this is done weekly on a specific day, or perhaps a certain delay occurs; I do not know, but there is a certain delay, which can reach five days and, possibly, longer.
I have some indirect evidence that the detail pages are updated once per WEEK, but I still control this and will report when my second update happens. At the moment, one page of the application is about five days behind, so I will send a message here when it is updated again, and will indicate how the current data at this time.