DSXchange

I encountered the problem:
the destination 'input_ava_credit' is already bounded

I solved the problem by change this:
if NEW=1 then if
NullToZero(LK_JOIN.CREDIT_AMT) <> NullToZer(LK_JOIN.CREDIT_AMT1)
then 0 else if
(NullToZero(LK_JOIN.OCCU_CREDIT) <> NullToZero(LK_JOIN.OCCU_CREDIT1)
or NullToZero(LK_JOIN.AVA_CREDIT)<> NullToZero(LK_JOIN.AVA_CREDIT1)
or NullToZero(LK_JOIN.LAST_CREDIT)<> NullToZero(LK_JOIN.LAST_CREDIT1) )
then 1 else 2 else 3


to

if NEW=1 then if
NullToZero(LK_JOIN.CREDIT_AMT) <> NullToZer(LK_JOIN.CREDIT_AMT1)
then 0 else if
(NullToZero(LK_JOIN.OCCU_CREDIT) <> NullToZero(LK_JOIN.OCCU_CREDIT1)
or NullToZero(LK_JOIN.LAST_CREDIT)<> NullToZero(LK_JOIN.LAST_CREDIT1)
or NullToZero(LK_JOIN.AVA_CREDIT)<> NullToZero(LK_JOIN.AVA_CREDIT1) )
then 1 else 2 else 3

It means that I just changed the order in the condition,specifically the last two 'or' logic order and it works.
My question is why all this happened?a bug or something else that i do not know?