I don’t know what this image is supposed to tell us, but I can confirm that KCalc behaves badly in some common situations. (At least, it does in Plasma 5.) Want to see an example?

Put it in Simple Mode, and try copying and pasting various multi-digit numbers with leading zeroes. Some of them work fine. Others, like 054 and 009, yield surprising results.

A programmer or mathematician might be able to deduce that KCalc is trying to interpret those numbers as octal (base 8 instead of base 10), if they’re paying close attention. That doesn’t help anyone who is just trying to total a bunch of numbers from a document, using their default desktop calculator, and doesn’t notice a misinterpreted value along the way. Their total will just be wrong, or in the case of nan, they will just be frustrated that the calculator doesn’t work.

This behavior is probably not appropriate for Simple Mode.

It does the same thing even in Numerical System Mode with decimal (base 10) explicitly selected, which is absolutely not appropriate.