Wrong typing position after highlighting

This happen randomly while typing which caused me a lot of typos.

Imaging I have this sentence on my screen

"I am using iMac"

then I highlight the "am" in the sentence and try to type "was" the replace the "am"

"I am using iMac"

after typing "w" i should be expecting the following

"I w| using iMac" ("|" is the current typing position)

however in some instance the typing position would be in the left hand side of the "w"

"I |w using iMac"

so when i type "was" after highlighting "am" i would have typo like the following

"I asw using iMac"

this is annoying, how do i fix this?