Does anyone know why ReadWriteStream overrides #contents from WriteStream?

WriteStream behaves as I would expect

|stream| stream := WriteStream on: String new.

stream nextPutAll: 'chris'; reset; nextPutAll: 'C'; contents "---> 'C' as expected"

but ReadWriteStream doesn't...

|stream| stream := ReadWriteStream on: String new.

stream nextPutAll: 'chris'; reset; nextPutAll: 'C'; contents "---> 'Chris' unexpected!"

I want to reuse a ReadWriteStream, so I want #contents to honor the end position. What's going on here?