On the document of the map of the route that they took shows the strait that they were going through which was about 400 miles. Magellan navigated the way by drinking the sea water to see if its more fresh.
On the document that mentioned the battle in the Philippines wasn't part of Magellan's mission in the first place so he didn't really focus on the task he had planned.
Magellan and his crew were running low on supplies so they didn't have enough food. So he probably just wanted to make the food last a little longer.