A little digging through the posts here has provided some insight to my question. The reply to 'Time to Replace RaSTI' titled 'STI calcualtion in statistic mode' has highlighted the part of the manual ...
If RaSTI is defined as a single figure value and is also calculated by examining the MTFs to the 500Hz and 2kHz octave bands why does EASE force us to map 3 separate values at 500Hz, 1kHz and 2k ...