Check the containers in the character's inventory (backpacks, etc, and there is a pouch container with the the same icon as a pouch of moondust or bonedust). It is likely gold added to their inventory is being stacked with gold in a container (possibly a crate picked up without looting first, etc). This also applies to gold transferred from other characters to a trading character when clicking on the balance icon to try to buy items; the gold is added to the container, rather than the trade window.
