Each problem is a short expression - three numbers, two operations - and the answer depends on doing multiplication and division before addition and subtraction. Every expression is generated so that working straight through left to right gives a different, wrong answer.
This page starts at the fourth-grade level: two operations, no brackets. Set Expressions higher for three operations, then parentheses, then a squared or cubed term - the progression from fourth grade to sixth.
Parentheses first, then exponents, then multiplication and division from left to right, then addition and subtraction from left to right. The last two pairs are each done left to right, not one before the other.
It starts in fourth grade with two operations and no brackets, and runs through sixth grade, which adds parentheses and exponents.
Because the expressions are built that way on purpose. 2 + 3 + 4 is 9 whatever order you use and tests nothing; 4 + 3 x 2 is 10 done right and 14 done wrong, and that is the kind this page generates.
Yes, and never negative. Division always comes out even. The sheet is for students who have not met negative numbers or fractions in an expression yet.