And it will go even lower as people start relying mpre on AI...

G U I N U S.

I know it's probably a typo, but I'm enjoying it.

Because its not really "1 plus 3", its negative 1 plus 3 which is two. I know it seems a little weird but the minus sign is " tied" to the thing following it.

Parenthesis, exponents, multiplication, division, addition, subtraction.

should actually be

Parenthesis, exponents, (multiplication and division), (addition and subtraction).

Addition and subtraction are given the same priority, and are done in the same step, from left to right.

It's not a great system of notation, it could be made far clearer (and parenthesis allow you to make it as clear as you like), but it's essentially the universal standard now and it's what we're stuck with.

Addition/subtraction work out the same regardless of how you order the operations. If you do subtraction last you start with the original:

9-1+3

and you are adding 3 to the result of (9-1). Since you are trying to perform it before the (9-1) operation is carried out, you can add 3 to the 9:

12-1 = 11

or you can add three to the -1 and get:

9+2 = 11

You only end up with 9-4 if you were subtracting 3 rather than adding three. It all becomes more obvious if you read the original as:

9 + (-1) + 3