Maybe one is listing US sizes and one is listing UK sizes?
Nope, I'd say they just listed them wrong.
2, 4, 6, 8, etc.. (i.e. event numbers) are US women's sizes. The UK uses the same series of numbers, though they might be sized slightly smaller.
1, 3, 5, 7, etc... (odd numbers) are US "Junior's" sizes (typically smaller through the chest and hip.. cut narrower for smaller/younger builds). I don't believe there's any gear manufacturer that actually sizes things using this numbering system (not in jackets anyway - I do believe both Fox and Alpinestars casual wear is sized this way).
That said, like ItGirl said, Alpinestars does run petite.