The human mind maintains lots of data and even more indexes to that data. One is an index of words that point to their spellings. When we think of the word "There" our brain takes the sound of the word there and looks it up in its spelling index. If you are thinking using individual words your brain will find the spelling of "there" and say.. "NO move to the next spelling of the word". It will find the "their" spelling in the index, know it is correct and use it.
If you are not thinking us individual words and are just typing a complete phrase or sentence as a single thought.. your brain will not object to the first spelling found and you will type "there" instead of "their".
My brain has "their" and "there" filed in an opposite order to yours. I am constantly typing 'their' when I should type 'there'.
It is a very common problem of those who think in complete phrases and sentences rather than in individual words. People who do not think using complete phrases and sentences do not have the problem.
Well I never thought of that. You're right, the brain does process and file a lot of information, some brains more than others.