A book about KAL007 suggested that KAL pilots were getting bonuses for saving gas. So they cut a corner off the route. We don't know how many times that was done.
A 747 can zip across Kamchatka and Sakhalin in next to no time.
The Russians may not have been ready to intercept over Kamchatka, but once alerted they were likely waiting for KAL007 when it got to Sakhalin.