You know what this is.
As it turns out, iOS 11’s Calculator app doesn’t work. it actually lags to such a degree that it will skip over key presses if they’re pressed too close together. So for example, it will miss the second “+” in 1+2+3 and give you an answer of 24.
So, I guess if you need to calculate something, you’d be better off with a browser-based calculator. Like this one. You know, if you’re an Apple user on iOS 11.
Source: lifehacker