With Dynamic partitioning, Hive would scan default partitions in some cases even if query excludes it.
As part of partition pruning, predicate is narrowed down to those pieces that involve partition columns only. This predicate is then evaluated with partition values to determine, if scan should include those partitions.
But in some cases (like when comparing "_HIVE_DEFAULT_PARTITION_" to numeric data types) expression evaluation would fail and would return NULL instead of true/false. In such cases the partition is added to unknown partitions which is then subsequently scanned.