Question about prioritization state management

I found the example in 5.3.4 Prioritization State Management <http://http2.github.io/http2-spec/index.html#priority-gc> very confusing.

In the previous paragraph, it says

Resources are shared between streams with the same parent stream, which means that if a stream in that set closes or becomes blocked, any spare capacity allocated to a stream is distributed to the immediate neighbors of the stream.

And in the following example:

Prior to the removal of stream A, if streams A and D are unable to proceed, then stream C receives all the resources dedicated to stream A.

It seems to me a bit contradictory, since A and B are neighbors with the same parent stream, then why doesn’t B share resources dedicated to A?

I know this question may seem dumb, but I do hope to get answers from the experts. Thank you.