More curiosities with the GTA!
It turns out that not only Highway 400 was supposed to be extended south, exit numbers reflect it. Now I know why Highway 401 is Exit 21 instead of Exit 1; it reflects the southward extension.
The street network is odd - streets shift and turn, the grid is skewed, and some streets don't even have directional suffixes. Vaughan and Markham don't seem to have this problem, though isn't their street network a continuation of Toronto's? Maybe Toronto's street network dates to the colonial era.
The reason why Markham and Vaughan's street network is a continuation of Toronto's is because present day Toronto used to be part of what we know now today as York Region. It was York County way back when, but the point is, that Toronto used to be part of what is now known as York Region but split in 1954 to form Metropolitan Toronto. Metro Toronto would exist until 1997, in which the six cities which made up Metro Toronto were amalgamated into a single City of Toronto. The only part of Toronto that's any bit confusing to me if you want to be honest is East York.
A slight curiosity and fun fact, Eglinton Avenue passes through all six former cities during its length across the city.
If you notice, the street grid in Peel Region, that is Mississauaga, Brampton, and Caledon is roughly diagonals instead of N, S, E, W, this concession grid was drawn parallel to Lake Ontario I do believe. The point I'm making is that it's not the same grid system in Toronto/York Region.