If you see the deduction with a note "Deduct Credit-AuditWithdrawal" or "Credit Out-AuditWithdrawal" following your withdrawal, the deduction is the removal of the credit bonus.
Each time you made a withdrawal, the credit bonus will be deducted proportionally to the withdrawal amount.
Please check this link
for the T&C of the deposit bonus,
and refer to article no. 14 which is written as "If you opt to withdraw your original deposit, a partial/full trading credit will be removed from your account accordingly."