There have been only 2 occasions since MMP has been introduced in New Zealand that an incumbent government that had lagged in a poll has been re-elected.
I don't think this statistic means anything near as much as you think. There have also only been three occaisons since MMP was indtroduced that an incumbent government has lost under
any polling condition. So there having been two separate times that polls have incorrectly predicted an opposition win in that time span is a fairly bad statistic to have if you're the opposition.