As a native speaker who types around 70 words per minute, I can tell you that when I type, I "think the words" in my head and it comes out my fingers. I will very often type "whose" when I mean "who's" and "your" when I mean "you're" and other similar sound-alike errors because my hands can't go as fast as my brain but they are trying madly to transcribe what I"m thinking. I think I've even typed "here" for "hear" for the same reason.
I almost never make these errors when writing longhand, I guess because it's a different process - my brain MUST slow down to not get ahead of my slow-moving pen.
I'll take "your" as a mis-typing of "you're" over "ur" ANY day!