Either they've spread, there's more than 1 of them or they're a multi-national corporation. Westfield in the past decade bought out a lot of the shopping malls in the San Gabriel Valley, they also seem to enjoy naming them all "Westfield Shoppingtown + Name of City." Case in point...the West Covina Fashion Plaza changed to "The Plaza at West Covina" and finally in its current form, "Westfield Shoppingtown: West Covina" when the former owners sold the property about 3-4 years ago.
Funny thing is...no one around here wants to call it that. It's still "The Plaza," around here. Who wants to call it "Westfield Shoppingtown?" Bleeeeh. (At least they've left the Eastland, another West Covina shopping center's name alone...then again it's not really a shopping mall anymore)