The very first snapshot of an RDS database contains all of the DB's data. Subsequent snapshots are performed incrementally, but from your perspective a snapshot is a point in time view of the entire database, not just the delta.
The restore and export processes result in a complete point-in-time image of the database and you get all of the data as it existed at the time of the snapshot. AWS handles this transparently for you.
So, if you export the data at time T and again 1 month later then, yes, the latter export will contain the data that was present at time T modulo any changes that were made to the database in the following month.