Alright this post spells it out perfectly:
http://www.pprune.org/8379267-post4191.html
The arc is in fact from the last ping.
That being the case there must be other arcs from the other earlier pings we are not being shown. If the plane was just a bit farther north (or south really) when they lost contact these other arc might be such that one of the N/S duplicates would be impossible. They must have pings from when they actually knew where the plane was. It is an unfortunate coincidence that the plane was so close in the N/S direction to the satellite when they lost track of it...