My guess is that April 1, 2015 being the start of current fiscal year is hereby being considered as the beginning of the new fast track processing (less than a year) as mentioned by CIC earlier that from 2015-2016 the processing times for a citizenship application will be reduced to less than a year, therefore I believe that all the applications received by CIC on or after April 1st (whether accepted for processing by CIC under old rule of 3/4 years or after June 11th under 4/6 rule) will now be processed in less than a year while all the old applications that were submitted before April 1st too will be cleared by March 31, 2016 at the latest, contrary to the processing times taking longer than a year in the past, thus leaving no old backlog anymore by March 31, 2016 and all new applications will be processed in less than a year.
March 31, 2016 has been referred to as the end of current fiscal year (see below) which is why they gave the cut off date for applications submitted by March 31, 2015 i.e. end of last fiscal year. Whereas applications received on or after April 1, 2015 are being considered as new applications (under the new fast track program) and received in the current fiscal year. Hope that helps and hope my understanding of the below is clear too.
Quote "New citizenship applications are being finalized in a year or less, and it is expected that the backlog of older files will have been eliminated by the end of this fiscal year. Individuals who submitted a citizenship application before April 1, 2015 will have a decision by March 31, 2016." Unquote