Why are some write requests throttled whereas the average consumed write capacity is below the provisioned write capacity?
It doesn't have to be a hot key or partitioning issue where the total provisioned write capacity is spread across the partitions.

You have a 5 minute average chart there. Technically you could have a case where you do have zero write activity for a 4:50 minute period and then have 10 seconds of ridiculous amounts of writes. These will most likely exceed you provisioned writes while the average still is below the limit.