cost change at sub inventory level

hi,

we are using ave cost in R12.1.3. would like to understand cost changes of item1 sitting in a costed org but in different sub inventories.

item1 in orgX with 2 sub inventories

SI-A has qty = 6000 (costed)

SI-B has qty = 6000 (uncosted)

total = 12000 in org X

cost = 1.

we then performed transactions SI-B (misc.issue) of qty = 1301

new cost = 0.7

again performed misc.issue of all remaining qty in SI-B

current cost of item1 = 0.00

when you check cost history of this item the above transactions are not there but appear in material transactions however the item cost has changed.

please assist explain the logic