those other vlans/subnets were probably already on the switches and may have been used for extra capacity, security separation, or even something like internal

those other vlans/subnets were probably already on the switches and may have been used for extra capacity, security separation, or even something like internal backup use you got lucky though, most of the time the entire network of shared hosting providers is blacklisted from sending mail. they may have just rolled out a brand new subnet or something