OK, here's my personal hypothesis, as I have no idea what are each company's credentials for such title, but here we go:

If, for instance, you have 10 years of professional experience in a specific tech stack, let's say front-end development, you have mastered your craft, and you deliver faster than your deadlines, my personal opinion this should sound like a senior developer's level of expertise.

I could be wrong though, but I'm talking from experience that I have discussed such topic with acquaintances of mine.