The number and frequency of sailings to USA shows how lucrative the transatlantic trade was, with various routes served by competing shipping lines. There were regular sailings from Southampton, Glasgow, Queenstown (Cobh of Cork) and other ports, but not all travellers to America went via American ports such as New York and Philadelphia. The alternative option, especially if your destination was a northern State, was to travel via Canada.
It is noticeable from passenger lists from the 1890s and 1900s where the ship was sailing to, for example, Montreal via Quebec, that the “port at which passengers have contracted to land” field is being used for landlocked locations: in other words, places such as Chicago IL or Detroit MI to which a large sea-faring vessel could never have sailed!
It seems unlikely that the shipping line would have troubled to collect information of no significance to it. It therefore seems probable that in these cases the passengers had bought an inclusive through ticket, covering both their transatlantic voyage and their subsequent overland journey by rail, or possibly road, to their final destination.
The snippet from an 1890 Montreal-bound passenger list shows a passenger going on to Spokane WA and another heading to Calgary AB on the CPR (Canadian Pacific Railway).