Columbus also was one of many who had fallen for the ballyhoo of Marco Polo, who drastically exaggerated the size of Asia to make his adventures there and back seem all the more heroic.
So Columbus believed the earth wasn't as big around as actually it is at about 25° north latitude. And he also believed Asia extended far further to the east than it actually does. Between them, he underestimated the sailing distance to Asia by about 5000 miles.
Which should have been a fatal miscalculation because his ships could only sail about 3500 miles before running out of food and water. So the typical strategy was to sail 1750 miles from land and if they hadn't found anything, turn around and come home. But Columbus sailed about 3100 miles before he made landfall in the Bahamas.
So if he had not been rescued by a completely theretofore unknown continent (or two), he'd would have never been heard from again. Instead, pure blind luck (and some guts) made him immortal.
My governor would have branded him a knucklehead.
Columbus encountered the Bahamas, Cuba and Hispaniola on his first voyage. He only saw bits of South America and North America on later voyages.
Columbus didn’t think that until his third voyage, per the Wikipedia link you posted.
Such a model had first been theorized by Christopher Columbus on his third voyage. Making observations with a quadrant, he “regularly saw the plumb line fall to the same point,” instead of moving respectively to his ship, and subsequently hypothesized that the planet is pear-shaped.[9]