I just started reading "The Bourne Supremacy" and I see that in some places Robert Ludlum spells things the British way, and in others the American way. For example, on one page you'd see "favorite" and on another one you'd see "favourite". Or, he spells "color" like "colour", even though he's American. There are also much more things that I won't include here.
Is that a common thing in books written by American authors? Or does this style of writing seem weird?