Traffic Light Thought — Stupid Speed Limit or Stupid Signs?

I wonder why there is about 650 metres of “open road” (100km/h) on ramp between 50km/h Onewa Rd and the 80km/h zone approaching the Harbour Bridge?

1. I join the on ramp from Sylvan Avenue.
2. About 150 metres (estimated) along the on ramp a sign indicates Open Road and means that the maximum speed that you can travel at is 100km/h.
3. Another couple of hundred metres later you get a warning that an 80km/h zone starts in 300 metres.
4. The 80km/h zone (which is actually practically far less than that about 6 hours a day due to traffic!) begins.

Why not just go from 50km/h to 80km/h?