I think what you are referring to is "temperment", and a TLDR version of that topic would be that for tuning a piano so that it can sound in tune no matter what

I think what you are referring to is "temperment", and a TLDR version of that topic would be that for tuning a piano so that it can sound in tune no matter what key, it is tuned to be evenly out of tune everywhere, aka, "equal temperment". This little app follows equal temperment. A string player playing unaccompanied solo might tend to play in a way that sounds better, aka, "just temperment", and so would seem to be out of tune according to this tool.