It's done on purpose to see if any major issue pops up. Be thankful that the developer beta serves as a testing bed before you get the (same) build that's confirmed to work without bricking your device.
PB's usually roll out on the same day closing in on the release due to the builds getting more and more stable/GM-tier.
Next DP will probably have the PB at 1PM PDT. DP6/PB5 will probably both be 10AM releases.