Working for Microsoft, and moved to Seattle, WA last year, after having lived in Vancouver, BC for about 1.5 year.

I can confirm this trend for several reasons:

1) The pay gap is real. Many get about a 40% pay increase by moving 3 hours down south. It's significant.

2) Taxes are aprox 10% lower in Seattle. Makes a difference too.

3) Rent-net-pay-ratio is about the same for Vancouver/Seattle

4) For a tech worker the health care is both places privately insured and the same.

Personally, I miss Vancouver. The city is awesome, and life quality is better, but financially it's simply not attractive enough.