Why do roads have more than one name?

The other night we were out touring Guelph and Kitchener. We ended up at a Swiss Chalet in Kitchener and had a delicious 1/4 chicken dinner - it was the festive special, with stuffing, cranberry sauce and even chocolates... mmm
Oh, yeah, my blog...
Anyway, we turned of this expressway thing onto a road. I had a map, and because it was dark we were proactive enough to bring a flashlight. The road name was no where to be found on the map, yet we were driving on it.
So we asked the server at the restaurant, she was very helpful in naming all the streets and how we could get to here and there, however, she as well was unable to locate "us" on the map. You guessed it, a paper map. Yes, a GPS would have come in handy with a little "you are here." Why thank you!
A little later back home with full stomachs I googled and zoomed in on the town - voila! the answer... The road also had a 'highway' number, my map had a number, but not the street name, the street had the name, but no number.
There it was, right on the map the whole time.