US Stamps With Canada Cancellations - What's The Deal?

These US stamps have Canadian cancellations. This happen often?

Possibly someone sending a letter from Canada and looked up US postage rates and figured if its goin there why not use their stamps? I almost sent a sase with canadian postage to the US to have it sent back and but realized it would have been a bonehead move. However since so many things seem to make it through Can post and USPS it might make some interesting covers for future generations. Now I'm thinking of seeing if it'll work.

As for the first example, sometimes on incoming international mail Canada Post will send the cover through a sorting plant to get properly shuffled to it's correct destination. While in the plant's system it does get canceled by the two or three lined spray-on ink-jet cancels. If you had the left hand side of the cancel you would see that the postal code, when looked up online, is most likely a sorting plant near a major airport, and thus a customs centre also.

The second one, an older slogan cancel I am not as sure about. Perhaps the same thing happened but only because the stamps did not arrive in Canada fully cancelled at that time and someone thought to shoot it through the machines to get it cancelled and sorted.

But, then again, on some pieces of mail I have received, there are no Canada Post cancels, just the originating office, and if trusted to machines, then no cancels at all. Happens on larger pieces of perhaps non-machinable mail, or it does get cancelled, just not where the stamps are.

Besides, the US$ and the C$ are practically equivalent nowadays

Would this US Scott#1423 1971 ewe and lamb fit in here?

That is nice.

I suppose some of these (supposing here) could have missed being cancelled in the US and someone noticed when it arrived here and thought to (and had time and permission to) cancel them.

Or someone could have mailed it that way from Canada too I suppose. There are always the exceptions for everything seemingly.

How about U.S. postmarks on Canadian stamps. 1909 Seatt;e, WA to Centrailia, WA with Bomar S09-06 expo cancel on Canadian 1 cent.