I have conferred with colleagues about this and the consensus is to carry on with what you have. In other words, this rule may be interpreted to mean the contents in the current volume. In volumes with no sub-items, use 1-3 and only in volumes where there are sub-items, use 1-5, 3-7 as you discuss. This is what most transcribers do and it makes the most sense.
This would be my own recommendation and it is, in fact, what I do in my own transcribing.