Game Mechanics: funny math

[Akfar]

I am in oggok today making feild point arrowheads for skillups on gugruk and selling them to the vendor Zarg Knightbane. he gives 5c per arrowhead, thus 1g per stack.

I sell by the stack it says you receive 1 gold from Zarg Knightbane, but I actually recieved 9s 3c. If I sell them individually instead of by the stack I get a full 1g.

With that in mind I proceeded to sell stacks of water to see what happens.

Zarg Knightbane tells you "I'll give you 9 copper per water flask", which is true.

9c per flask X 20= 1g 8s,which the message said, but when you sell by the stack he gave me 1g 8s 6c

Proceeded to log on akfar in felwithe went to merchant Irontree.
"I will give you 1 silver per water flask"
"You recieve 2 gold from merchant irontree"
Yet what i recieved was 1g 9s 1c
Repeated with the same effect

I did notice a post in the search about being overcharged per stack but nothing about selling stacks either giving too much or too little.

Guess I do everything individually now