There are probably a lot of factors in play here; a few of the biggest that I recall from looking into this a while back:
 
1) EWEB has huge capital costs associated with its hydro dams (Carmen-Smith, Leaburg--Walterville), while SUB buys all of its juice from BPA, so no fixed capital costs to cover;
2) EWEB has a progressive rate structure that encourages conservation and imposes a surcharge on residential customers using over 800kWh/month (SUB has a tiny surcharge that starts at 1,500 kWh/month during the heating season). This can be a real hit for folks that use electric heat during winter and have poor insulation.
3) EWEB has a much bigger distribution network than SUB with higher maintenance costs, hence the higher delivery charge as part of their electric rates.

I wish someone from EWEB would provide a more accurate/ up-to-date information on this; it's been a while since I looked into this issue.