My tax liabilities report is different from my Bas report

My tax liability report says I can claim $1079 from purchases. But when I configure the BAS report it says $1238. I have checked the codes on Purcheses - They are all NCG of NCF and on Sales they are all GST. The Sales figures are all definitiely corresponding with each other but how can the Purchases not be the same please? I wish I could give you screen shots but I don't know how to do that basic stuff. I have been using QuickBooks for years and currently have QBooks Plus 2012/13.  Pat.