Since I just made an aftermarket instrument cluster for the GTA, I'll share what I think may be your problem. First off the voltmeter is probably the most reliable gauge of the 26 year old gauges. Tying in to the dash connectors is the printed circuit on the back of the gauge(s) housing, it's a press fit deal and you can see that it could easily suffer from a poor connection, secondly the factory uses some measly 22 gauge wire for most of the gauge feeds and since the gauges are at the end of the line one could assume it can only be inaccurate from age and design.
That being said, the voltmeter is past the gauges fuse so it really only measures the alternator's output in V when the car is running,,, not running, it's reading battery voltage on the "key on" position. Remember the battery is still just storage reserve load, so I would check at the battery voltage key on/engine off, then check the gauge reading at the same time. I would chalk up any difference as line loss and poor connections with a second concern to be a faulty gauge. That is if the battery is considerably higher in voltage.
If this still bugs you, and it would me too, I'd pull the instrument cluster out(white housing) and check the C1-# pin connection with key on/engine off to see if a higher battery voltage is present. Next I'd clean the printed circuit's bare copper trace with a pencil eraser to clean off oxidation and see if that brings the gauge back up to a closer reading . Also if you go that far into it, there are 2-3 grounds used on that printed circuit and there not divided up as you'd think, one may be for speedo/tach lighting and some of the GI lighting while another is for voltmeter/gas gauge and different GI bulbs. But a factory service manual has the pin outs and test voltage needed to be present. So check the ground connections too.
