Hm. I don't think there's a good explanation for that. It looks to me as if Kibana just displays the source fields in the order it receives them from Elasticsearch, and that ordering is opaque (probably varies based on various algorithms). I think there's an argument to be made that this is a flaw, and that Kibana should sort these, so they always display consistently.
You can open an issue here:
https://github.com/elastic/kibana/issues/new/choose
If you do, please be sure to note the version of Kibana you're using.
Best,
Chris