I notice that in maps that are both published in the early 1900's or maps that refer to that period but published in the modern day have this label. I can cite some examples if that is necessary. I thought that when the Ottoman's conquered Constantinople, they renamed the city immediately so why is this reference made? I have tried to look through wikipedia and couldn't find an easy/obvious answer, could someone give me a reason as to why there was a lack of naming cohesion?
The official name change happened in the 20th century! Kostantiniyye and Istanbul had traveled side-by-side as overlapping names with sometimes the same, sometimes different uses for centuries. Here are a couple of useful threads. We should probably add this to the FAQ...