@Dinger and Cyborg
Dinger, Cyborg is saying that the altitude or whatever s*** it is at Safeco is suppressing Seager's fly balls from leaving the yard. This is not accounted for by park factors in the same way that a fly ball hitter at Coors benefits more than the average hitter from Coors. They're being compared to the same run scoring environment (if I understand park factors correctly), but one player has a leg up (or down in Seager's case) on the other just due to the park factors being more of a proxy than anything.
However, Cyborg, I think it would be interesting to look into if it would actually be that big a boost. I don't think 130 wRC+ guy is accurate. Theoretically, if fly balls are suppressed in Safeco, then pitchers would induce more fly balls on Seattle hitters. So in a neutral park, Seager would have more fly balls leave the yard, but fewer fly balls in total.
however, if pitchers don't actually behave in the expected manner, then you would have something exploitable in a variety of ways.
I don't know if my interpretation of park factors is right. I'm assuming it's just a player's runs compared to (above) what you would expect in the run scoring environment. Overall, not broken down into parts.