The Garrison Sgt Maj and (I think) the RSM of the college at Sandhurst and (at least) another have special terms of service which mean that they serve for about 10 years, having given up their opportunity to be commissioned. However, part of the deal sees them actually holding a commission which is 'in suspension'.
It is said to be a huge jump from being 'top soldier' to being just another Lt/Capt etc. Some on this site will recall the RAF Regt WO Danny Gould (sp?) who was commissioned and I understand found it difficult to adjust fully - perhaps it's best to be top of the heap rather than halfway down the pile!
As to an attendance at Staff College, there is probably a good reason for that.
Old Duffer