Loss of Archivelog on Primary causes issue for RMAN on backup of Secondary

I had a situation recently when backing up an oracle that an archivelog had been removed from the primary but was present on the secondary.

a)  I was using a catalog

b)  Data Guard Physical standby situation

c)   I was backing up the Standby

d)  Archivelog missing from Primary

e)  Archivelog Present on Standby

f)  Backup of Standby gives error message that loss of this log compromises recovery when it was present in standby but not on primary.

Question - if log is present on seconday and you are backing up secondary, why does rman care (or even know) that the log is missing from the primary.