They're more "North/Central" than "Eastern" Washington too, though it seems like everything more than 30 miles east of Redmond is counted as "Eastern" by a lot of folks.
I wouldn't count 98% availability as "good" for electric service. In 7 years I've been at my current location, we've accumulated a total downtime of about 4 days - 3 of those during a MASSIVE ice storm that covered several states a couple years back. More years than not have been ZERO downtime. I've seen similar levels of availability everywhere I've lived in the US - 99.5% or better is the NORM here in recent years.
People from around here consider themselves in Eastern Washington, it is more the cultural identification. Personally we have not had any downtime by the PUD (knock on wood). With that said, we are on a high availability grid so that may have something to do with it.