Amazon SimpleDB keeps multiple copies of each domain. When data is written or updated [...] and Success is returned, all copies of the data are updated. However, it takes time for the update to propagate to all storage locations. The data will eventually be consistent, but an immediate read might not show the change.

Consistency is usually reached within seconds, but a high system load or network partition might increase this time. Repeating a read after a short time should return the updated data.