Why do words like "salah" and "riba" have an extra unpronounced letter waw in them?

I've noticed that in many masaahif ("mushafs"), certain words like "salah" and "riba" have an extra, unpronounced letter waw in them, such as Surah Baqarah, verse 3: The fifth word here is pronounced "salah," although the waw with the small alif above it is pronounced as an alif, not a waw. What's up with that? I recall one of my Arabic teachers mentioning that it's not permissible to rewrite words in the Qur'an (for cases like this -- removing silent letters), because the sahaba all agreed to write it this way during the time of Uthman. (That's tangential, it doesn't need to be discussed here.)