I'm happy to report that in this update, I added the Open-Dyslexic font by Abelardo Gonzalez. Its bottom-weighted characters are designed to reduce letter-swapping and increase differentiation between similar-looking letters, which improves readability for people with dyslexia. It's now the bottom-most option in the font list in Instapaper's text-controls ("aA") panel.
So why I had found it easier to read from my iPhone? First, an ordinary page of text is split into about four pages. The spacing seems generous and because of this I don't get lost on the page. Second, the handset's brightness makes it easier to take in words. "Many dyslexics have problems with 'crowding', where they're distracted by the words surrounding the word they're trying to read," says John Stein, Professor of Neuroscience at Oxford University and chair of the Dyslexia Research Trust. "When reading text on a small phone, you're reducing the crowding effect."