Not all NOAA stations have this issue so it must be an artifact of the program/software your local weather service is using to create voice from text. I believe there may be a setting that can be adjusted to correct this issue or possibly a software upgrade.
Some of it may be due to buffering. Not hearing it first hand it is hard to say exactly. When there is a burst sometimes that will cause it to speed up or sound like it is speeding up. Then again it could be the software like was mentioned that renders the STT differently for certain phrases.