What can happen is this:
You have the standby database in READ ONLY mode and the primary is generating new archives. The archives are archived locally and remote, if possible.
A remote destination can be optional.
It could very well be that in some time interval the standby site is not receiving all archives and in a later interval it is receiving them again.
The FAL client will fetch the gaps during the recovery of the standby database and doing so, it will automatically solve this problem for you.
This is also a reason not to remove archives from the primary site, until you know absolutely for sure that they are OK on the remote site.
For me the only way to know they are OK is when they have been successfully applied on the standby.