Exceptions in "PDFBox_1_8_9_20150316_single_thread" but not in "pdfbox_1_8_9_20150316"

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SELECT File_Extension, count(1) as COUNT from comparisons where SORT_STACK_TRACE_A <> '' and SORT_STACK_TRACE_B is null and JSON_EX_A is null and JSON_EX_B is null group by FILE_EXTENSION order by COUNT desc