Wrong posting date in bkpf table

Hi I am facing a problem in account document post using FB01.I have created exit in GET_EXIT_TITLES with exit U100. in U100 i have added check that posting date should not less then last posting date. these all post in stored in table BKPF.its working fine when i run manully if i run using batch job then in some document its going wrong. means some document posting date is lower then last posting in BKPF. its happen sometime like 4-5 wrong form 100 document

Please suggest