Not necessarily true. People keep assuming that mainly because of the very first Public Beta last summer with Yosemite. More than likely because that was the first time ever they did PB and have since learned how to handle it.
Later betas have pretty much followed the DP getting a couple builds before PB showing up. And from then on in the cycle DP + PB have released together. (Makes zero sense to have PB people reporting bugs from an older build that could already very well be fixed in the "newer" DP. Thus why in more recent times they have released together once the PB was available.) Plus I would also assume there are more people on PB than on DP. Furthering the need for both to be released simultaneously.