When blue is typed over red, google dynamically updates the search.
But the browser hasn't been instructed to fetch a new url.
So, no blue roses url is pushed into the history.
Selecting a blue roses result pushes that into top of the history.
Back pops the last url, and you get red roses.

If blue is typed over red, and return pressed, the browser pushes blue roses into the history. Now the back button will work as expected.

Any who didn't experience the problem were likely hitting return without realizing it.