Have you ever added one, two and three using a calculator? Clearly, that makes you poor at math by the snooty standards, but you can feel at home here. I too have once (or more) probably. Now, the question is if you have ever done that “major calculation” on an iOS 11 calculator. If you have, then I would seriously suggest you to go back to that math problem, because your results were quite likely wrong. But don’t be hard on yourself, blame it on iOS 11.
Turns out, the calculator app on the new iPhone OS is a wee bit flawed. A Reddituser has recently pointed out that, when you quickly type 1+2+3 on the iOS 11 calculator app, you most likely wouldn’t get six as the answer. You might get 23, or 24, or 16, or 32, or something else, depending on what buttons you tap and in what order, and, obviously, none of which is the right answer.
Basically what happens here, is that when you type these numbers too fast, the app hangs a little, so instead of 1+2+3, you actually end up typing 1+23 or 12+3. Basically, you just get some or the other wrong answer. And it works similarly for other numbers too, so it is not like just the 1,2 and 3 are flawed. I tried adding 7+8+9 and got 89.
The Reddit user found that this issue was because of the new animation in the calculator app, where a button briefly fades to white when you press it. The result is that if you press an operator button (i.e., the plus sign) before the short animation finishes, the app ignores it. 
But you should know that this flaw occurs, only if you type really fast. If you type at a nice leisure pace, you are likely to get the correct results. Now, it doesn’t seem to be a deliberately done design that slows down the speed of the calculator, but you can never tell with Apple, and its “it is meant to work that way” justifications.
{BGR}