There's more than one possible reason. The two most obvious that spring to mind are:
1) You are running the P&L from 'Month 1' up to 'Month ' and there are transactions in the sales dated in the prior year. Try running the P&L from 'Brought forward' to 'Month ' and see if that makes a difference.
2) The sales on your P&L consists of more nominal codes than just 'product sales'. Look at the chart of accounts and see what range of codes is actually used for sales; the difference may well be the balance on one or more other nominal codes.
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Vince M Hudd - Soft Rock Software
(I only came here looking for fellow apiarists...)