I.e. "there" and "theater" are two different "th"'s.[3]
[1] https://en.wikipedia.org/wiki/Voiced_dental_fricative
[2] https://en.wikipedia.org/wiki/Th_(digraph)#Voiced_fricative_...
[3] English speakers: if you pay attention to how you speak those two words you'll notice that your tongue is doing different things to make those two "th" sounds. This is a weird "archaeo-linguistic" holdover in your "motor-culture". In a certain sense, "you" always knew this but didn't know you knew.