User blog comment:Dryuuu/The wonderful future of the two Js (and Merlin movie?)/@comment-5674726-20121127000757

Planning for only five series all along but telling us a Series 6 is a high possibility, then breaking the news there won't be after the first half of Series 5 has ended

Maybe they wanted the end of Series 5 to work as a show end, if it came to it, but that the series would end in such a way that Series 6 would be a possibility, if the Powers That Be decided to continue it.