How can the Database | Monitor | Database Probe show more % hit rate for 'overall', than for the 'default buffer'?
(Please note that neither 'keep' nor 'recycle' buffers are configured.)
"The "Default buffer cache" of the SGA is the default memory cache that stores data blocks when they are read from the database. If the DBA does not specifically place objects in another data cache, then any data requested by clients from the database will be placed into this cache. This memory area is controlled by the db_cache_size in Oracle 9i and above."
"The "Overall buffer cache" maintains statistics of buffer cache hits and misses and shows you the overall buffer cache hit ratio for the entire instance since it was started."
Considering this, it is quite possible, that the size of the "Overall buffer cache" over the time (since the instance was started) can be bigger than the actual size (representing the hit ratio of the moment) of the "Default buffer cache"
For further information please have a look at: