Difficulties in understading certain space figures associated to snapshots and objects with USEDREFR

Hello,
A colleague brought to attention the following issue, which lead to an "unexpected" storage pool exhaustion.
We'd appreciate if anybody could shed some light on it:

1. Assume, for simplicity, a fresh installed and fully updated  system with reasonable physical memory and just Solaris is running.
   (in my case LU was used to create and update an ABE and later delete the outdated GA2 image)
   
   
2. You, thus get something similar to?:
   
   # zfs list
   NAME                         USED  AVAIL  REFER  MOUNTPOINT
   rpool                       8.09G  11.5G  46.5K  /rpool
   rpool/ROOT                  6.29G  11.5G    31K  legacy
   rpool/ROOT/20120719-v1      6.29G  11.5G  4.12G  /
   rpool/ROOT/20120719-v1/var  2.17G  11.5G  2.17G  /var