I have had the same experience many times. I believe it depends on the route taken, sorting facility, and when your box is emptied, etc. Mail to my dad in Brampton is also often there in 1 day from downtown Toronto.
Of course, this is something that's nice to get, but not "guaranteed".
I have also often sent or received mail that arrived the next day. However, last week, I sent a first class item, from Mississauga to Toronto, that took a week to arrive. In the mean time I had mailed 3 other items to that same address, via the same mailbox, and they all arrived earlier than that one item. Why overnight for some items and a week for others? This is not the first time this has occurred.
It's usually pretty fast for me too. When I order things on Amazon they usually ship in th evening/night, and I get them the next day, less then 17 hours from Mississauga to Orillia. I actually find that UPS is slower, it takes them 2-3 days to get my items here.
Why overnight for some items and a week for others?
Probably if the item doesn't auto sort for some reason - Postal Code unreadable by the machine, etc, then it gets sidetracked to "manual". Make sure your postal code is in the appropriate place, on its own line and legible and you should normally not have such issues.
Like 57 says it should be legible, as:
To deliver the package, all they (Canada Post) have to be able to read is the House Number and the Postal Code.
The Postal Code gives them everything down to which side of the street, and in some cases the building, so the only thing else is the house number, and/or apartment number.
i love canada post. many times i've gotten packages in three days from richmond, bc to RURAL pei. staggering really. i've always preferred canada post for sending any smallish items. purolator for bigger items.