Arctic terns breed around the Arctic and winter in Antarctic waters, so the pole-to-pole distance alone is large. Until miniature geolocators became light enough to fit on a bird of about a hundred grams, the actual route was inferred rather than measured, and totals were underestimated.
When tracking was done, the recorded round trips proved much longer than a direct line. Terns from Greenland and Iceland flew down the Atlantic in a broad S-shaped path, pausing in the mid-Atlantic to feed, and returning north on a different track that used prevailing winds. Annual distances of roughly seventy thousand kilometres were recorded, and later work on birds from other colonies produced individual totals above ninety thousand.
The frequently repeated claim that an Arctic tern therefore sees more daylight than any other animal is a reasonable inference from breeding in the Arctic summer and wintering in the Antarctic summer, though it is not something anyone has measured directly. What varies between studies is the colony and the route, so no single figure represents the species.