2 + 3 × 4 is 14, not 20. Multiplication and division are resolved before addition and subtraction, whatever order you typed them in.
Precedence is a convention about what an expression MEANS, not an instruction about the order to press keys. A four-function calculator with no brackets evaluates left to right and answers 20, which is the right answer to a different question. The gap only shows up when the two orders disagree, so it survives every check that involves only adding, or only multiplying.
On a calculator that chains left to right - most cheap ones, and most phone apps in their basic mode. Also anywhere the expression came from a spreadsheet or a piece of code and has been retyped by hand: brackets are the first thing lost.
Estimate with the rule, then check it against the calculator that models it properly.
Open Scientific Calculator →2 + 3 × 4 is 14, not 20. Multiplication and division are resolved before addition and subtraction, whatever order you typed them in. Precedence is a convention about what an expression MEANS, not an instruction about the order to press keys.