Every word doesn't only have one meaning. Sampai Can Have many meaning depend on the context. Sampai can also mean Arrive, etc.
Sehingga can Mean different ways.
Sehingga in daily activities doesn't mean until. But it means "So". For Example.
I Study hard, So I pass the exam.
Saya belajar dengan giat, Sehingga saya lulus Ujian.