I also don't know why that image labels Canada and the US as if they're not part of the same continent. Sometimes Mexico down to Panama is labeled separately as Central America, but that's a regional geographic distinction, not a continental one.
As far as that goes, labeling Europe and Asia as separate continents is based on language, culture, history, and politics not geography - they lie on the same tectonic plate.
Some argue for just four continents, based on pure geographical distinctions - The Americas, Afro-Eurasia, Antarctica, and Australia. The line between geography and geopolitics often blurs.