Sam34.
I agree as it is a little confusing. In essence you are right 85% of the students pass their IR either 1st attempt or 1st attempt with a partial ( ie one section failed) which was passed on the retest.
The confusing bit is that the 15% that don't get a 1st series pass could all the 1st series passes with a partial that then failed the retest on the partial section!
Which ever way you look at it it's not bad, and most companies ( like CTC ) that specify a first time pass refer to a 1st series pass not 1st attempt.