You are not talking about the expectations in terms of the corporate levels. Staff and senior have clear definitions in tech companies. A staff(l6) is the team lead who is required to find the scope for himself and team. That's in his job description. If he can't do that he would get pipped or fired.

A senior(l5) is the tech lead on the project level. He is not in charge of handling the team's scope. As long as he handled his own projects well, he would usually pass the review cycle. The difference is crucial. At staff and above, the engineer's responsibility is vastly larger than a senior's.

I am not saying a senior can't do a staff's job - that's how he get promoted once he demonstrated that he is operating at a staff level, i.e "inventing" scope for his team. But a senior's scope is much smaller than a staff's.

This applies to almost all US medium to large tech companies