Hi Wei,
Please review SAP Note 1012874 - FAQ extractors 0FI_AP_* 0FI_AR_* 0FI_GL_*(except 10). This SAP Note contains very detailed technical information behind the various DataSources. Note that although your DataSource 0FI_GL_14 is extracting from New General ledger, most of the information should still be applicable in my opinion.
Question 3 might give an answer to you question too. Furthermore, have a look to the Safety Time (table BWOM_SETTINGS) which might explain why you cannot extract the changed documents immediately.
Best regards,
Sander