Korpisalo has been outplaying Copley, though, by both traditional and advance metrics. Since the trade, Korpisalo has a .921 save percentage compared to Copley's .915. Korpisalo has faced more high-danger shots than Copley (83 to 50) and has saved a higher percentage of them (.855 to .800).
Moreover, this is the list of Copley's opponents for starts since the trade (* for playoff teams):
Montreal (at home)
Washington (at home)
Nashville (at home)
Columbus (at home)
Calgary (at home)
St. Louis (at home)
*Seattle (away)
*Edmonton (at home)
*Colorado (at home)
Here are Korpisalo's:
St. Louis (at home)
*Colorado (away)
*N.Y. Islanders (at home)
Vancouver (at home)
*Winnipeg (at home)
Calgary (away)
*Edmonton (away)
Vancouver (away)
*Vegas (away)
Vancouver (at home)
Anaheim (away)
Korpisalo was given the more difficult schedule (more playoff teams, more of those games on the road) and still performed better.