HelpSILVER COPPER PROBLEM

ok this is my problem.. everytime i sell items in npc my copper and silver is 99. but after i sold 1 item to the npc my silver and copper became 5 silver 1 copper.. .WTF?? can someone help me?

IT BECAME GOLD......!! THE RULES IS
1 Gold = 100 Silver 00 Copper
1 Silver = 100 Copper
1 Copper = Nothing Really Lower Than Copper (Trash Maybe)

Note. Nothing Is Above Gold It Will Only Stack eg. 1000 Gold