That is a very good question!
You will notice that some airports have incredibly wide runway strips delineated by gable markers (EG Adelaide). NOTAMS can vary the prescribed RWS width on a daily basis so the gable markers dont' actually mean much.
I believe that it is up to the airport owner, but most of them look to CASA for guidance. The CASA inspectors do not agree across the board hence the variations. There was a watershed a few years ago and apparently what was accepted then is now audited as "the normal". If a change is required by the airport owner he has to produce a Safety Case to show that the risk of changing the strip width or removing the markers is acceptable.
