How do you make a survey?
Well, JB makes surveys that are advertised through the JB website/IDEs. These will favor JB users, especially the JVM, Python et Web ecosystems.
StackOverflow makes surveys that are advertised to all users. But the Kotlin community uses this slack much more than they use StackOverflow, so the Kotlin community will be underestimated on StackOverflow surveys.
Studies that are based on search results are biased towards languages that are difficult to work with. If you're working in a language that's very weird and inconvenient, you'll make a lot of Google searches, so it looks like the language is more popular. But maybe there's another language with 10 times the number of users, but it's so trivial to use that they just don't use Google at all. Or maybe they have all the documentation installed on their machine.
Etc, you can't really know anything for certain, it's all estimations and they're often not that great