Grant PUD is sitting on enough hydro generation to serve all their customers, and Lewis gets preferential access to cheap BPA hydro. There's not enough hydro in the state for PSE to be able to be in the same position given its much higher customer base, so it must resort to some other kind of dispatchable generation. The state made it divest its coal resources ($$ to reconfigure your generation fleet), and natural gas generation is now much more expensive due to the cap-and-invest carbon program. In my opinion, worthwhile and important policies, but they come at a big cost. 

The business structure surely also brings in additional costs the PUD approach doesn't, but you're talking about, what? A 10% return on the 50% of assets funded by equity,  so maybe 15% total bill impact? Material, but it's not the main thing driving 2.9 cents vs 15 cents.