Because they don't share the same motherboard? 😛
I have no idea in fact, but it seems logical.
For like $300 more in CAD, I don't see what's really wrong. Quad-core + Better graphics
Core i5 is new and it's pretty freaking fast... I've seen benchmarks and honestly, between the 2.66GHz i5 and the 2.8GHz i7, you're almost better with the i5. The i7 scores better in some, but the i5 also scores better in some and in the end, they're pretty equal. So don't pay that additional $220 CAD and just get the quad-core.