It looks weird from the outside, but yeah, we don't know how Zander did in training. Maybe Liam Kelly aced it and Zander just did okay. But from the outside it looks strange to have McLaughlin and Kelly in the squad ahead of Zander (who plays behind a similar defensive system and has played really well in big games since the Scottish Cup Semi).
And obviously it's not Steve Clarke's job to care about this, but because Jason Kerr's moved away in part because he doesn't think he'll break into the Scotland squad at Saints, it's incredibly frustrating to see Zander out of this squad especially when he's out of contact. It actively harms our chances of keeping players if the impression they get is of Saints harming their chances of a Scotland call up.
(On another note, you'd have thought Ralston and Watt would be in with a shout for this squad)