Because the two covered squares to the left of the “4-with-flag-on-top-of-it” can have only 1 mine (because of the “2” there), which means the “3” needs one more mine, which can only be on top of that “4”.
I see what you mean. That means that the covered square that is top left of that 4 has one chance out of 2 to be a mine. While the other two squares on its right each have 1 chance out of 4 to have a mine in them since they require a 1 chance out of 2 condition for it to be possible for a mine to be in one of those two remaining squares.
Because the two covered squares to the left of the “4-with-flag-on-top-of-it” can have only 1 mine (because of the “2” there), which means the “3” needs one more mine, which can only be on top of that “4”.
At least that’s my reasoning.
I see what you mean. That means that the covered square that is top left of that 4 has one chance out of 2 to be a mine. While the other two squares on its right each have 1 chance out of 4 to have a mine in them since they require a 1 chance out of 2 condition for it to be possible for a mine to be in one of those two remaining squares.