This is obvious, since u didn't submit ur previous I-20s, they need to know why the previous OPT was granted for? They won't let u have two OPTs for same level study. At this time they know u applied for OPT with master level study. So they wanna know whether the previous OPT was for bachelor's level or for master's level. If the previous was for master's level then your OPT will be denied but as u said ur previous OPT was for bachelor's level, no worry it will be granted.