Traffic Light Thought - The slower they go, the faster they change?

Why does it seem to be:

The slower someone goes down the motorway on-ramp, the faster they'll dive for the middle, "fast", lane once on the motorway?

Inspired by someone today who did 40km/h down the on-ramp, accelerated to maybe 50 km/h on joining the traffic (which was flowing at 80 km/h until they arrived) while cutting across 3 lanes to get to the middle. I wonder if the accelerator and indicators were linked as both didn't appear to work...