Stock is decremented and synced as soon as an order is placed on Faire by a merchant. However, you will not see this order until it is actually approved by Faire's internal process. This can sometimes take weeks if it's the merchant's first order and the order is large.
For example, an order you received in April may have already had its stock decreased for it back in March since that was when the order was first placed.
If an order is canceled during the approval process, its stock will be automatically added back in.