There are lots of variables so not really as good a test as it may seem. Traffic can make a huge difference eve when starting just seconds apart.
I use to model traffic in urban environments across a midlands city - predominately a major traffic corridor. It was done to compare the effects of air quality by running different traffic light phasing plans in parallel on a clone of the real traffic control system. We didn't always run a different plan especially when developing the system so often like for like but sometimes a few seconds apart. We found that even this small discrepancy in departure time could make a significant difference - 10% wasn't unusual.
Now I realise that this was an urban environment, but many years ago I have witnessed this myself - younger so no stopping involved just different driving decisions. I was travelling north about 200 miles, starting about 2 miles from the start of the motorway. A mate of mine was travelling further north but we met up and started together, We both got stuck in major traffic around Birmingham, but he hit it about 10 minutes earlier - it was an Easter weekend and this traffic hit the national news so exceptionally bad. By the time everything had cleared he phoned in and he had a huge advantage on us. Same time, same route, certainly would not have been exceeding speed limits significantly, but mostly different luck due to different driving decisions.