Ringing time lesser when set to Ring all at once

I did a test on incoming calls in 2 scenarios

Scenario 1, Ring in order, the number of ring set to 15 rings/75 Secs, going to voice mail after 1 min 50Secs

Scenario 2, Ring all at once, the number of ring set to 15 rings/75 Secs, going to voice mail after 30+Secs

Please can you advice why is this happened? and how to make Scenario 2 has the same duration of ringing time (1min 50Secs)