It comes from the type of tournament, to the best of my knowledge:
This page (
http://www.wizards.com/Magic/PlaneswalkerPoints/Information) has a good bit of information on the topic, and if the store wants to know more about the particular event, have them talk to their WPN rep.)
In particular, this table from that page is particularly helpful:
Here are examples of the different event multipliers available at events:
1x: Sanctioned Magic tournaments, Prerelease events, WPN Premium Qualifiers, Friday Night Magic
2x: Game Day events
3x: Grand Prix Trials, WPN Premium Tournaments, Public Events at Grand Prix
5x: Pro Tour Qualifiers, World Magic Cup Qualifiers
8x: Grand Prix
12x: (Lifetime Points Only): Pro Tour, World Magic Cup, Magic Players Championship