YouTube: Kirk walks the *other way* around the bridge - Star Trek Original Series, Doomsday MachineAnyone have a quick link to that Doomsday walk in screencaps?
As others have suggested, it appears that Spock was standing at the next station (Nav station), perhaps for lighting or other reasons relating to composing the shot (which means they must have moved his blue-light scanner-monitor thingy to the other station as well).