to be complete, what happened in 2015?

liberals: +20.5%. yeah.
ndp: -11%
conservatives: -8%
bloc: -1%
greens: -0.5%

so, the liberals were clearly seen as the consensus candidate on the left - they swung large amounts of (potential) ndp, bloc and green voters. and, they won a convincing, if somewhat weak, majority.

"but, they swung 8% of conservatives, too", you say.

this is a key number:

turnout: +7%.

so, that's actually a bit of a trick.