When traveling in the US with a Canadian SIM card, roaming does the following:
Phone → tower of US carrier → US carrier → SIM-corresponding Canadian carrier => Web page
The website has detected that the Canadian carrier is your connection source.
Alternatively, apps and websites might find you in different ways by using the location permission. However, there are intriguing other methods as well time zone, system properties. Apart from GPS, phones can use network locations with permission. Internet searches for the locations of neighboring MAC addresses are conducted using data provided by very accurate GPS-enabled devices. A Wi-Fi router or hotspot’s location is momentarily erroneous when it is relocated.
Very interesting, I appreciate your well-considered answer. Thank you. It is something I suppose I will have to put up with, but at least I will be able to comprehend all the advertisements if I return to Ontario.