One of these things is not like the others

One of these things just doesn't belong. See if you can spot the difference in the following single-election results - and consider what it might mean for each party's future strategy...

Vote Share	Seats	Provinces w Seats	Provinces under 20%	High Prov%	Low Prov%	Rebates
30.6%	103	8	2	42.9%	15.4%	306
25.5%	66	5	5	58.9%	3.9%	182
30.2%	103	9	1	52.5%	15.3%	283
29.6%	99	8	1	61.7%	8.8%	251

For those wondering, the parties who posted those totals are respectively the NDP in 2011, the Canadian Alliance in 2000, the Libs in 2006, and the Cons in 2004. And of course, each party served as the official opposition following the listed general election.

So let's ask the rhetorical question: is there an obvious reason why one of those parties might have had both a glaring need to pursue a merger, and an obvious opportunity in doing so?

And conversely, is there an equally obvious reason why the other three might see fit to work from an existing national base, rather than pursuing wrenching structural changes?