It's not purely the reason; it would be part of it. It's usually a combination of a number of factors. One is that they had other offences they were convicted of at the same time, which changes the profile of that individual. The eligibilities for release were changed in the legislation as well. They were eligible for parole halfway through their sentence. It changed the actual eligibility timeframe for them, and that would delay their release as well.
On April 29th, 2010. See this statement in context.