iOS 10.3.2 is just a minor update update while 10.4 is a major update which could be shipped alongside the 10"5 iPad Pro (2nd Generation). In the past, there were more than one iOS version in the beta testing.
Ehh thats not really true. Only time there were multiple versions was when iOS 8.4 and iOS 9 were in beta together, but that was for less than 2 weeks. They were just putting the finishing touches on 8.4 and waiting for the release of Apple Music before launching 8.4 publicly. The beta path strictly went to iOS 9.
iOS 9 Beta 1 = June 8th, 2015
iOS 8.4 Beta 4(GM) = June 9th, 2015
iOS 9 Beta 2 = June 23rd, 2015
iOS 8.4 Public = June 30th, 2015