Thankyou all for your input. This problem had me really stumped, however I have managed to solve it.
Everything seemed to check out, the same number of rows was appending, there was no rounding, there were no null values that were being omitted, and when I did a straight copy and paste, the amounts summed correctly!
But what was hapenning was that the original source file was in excel format. When I did the copy and paste it was from this format. However to maintain certain data type requirements we saved this file as a .csv so as to do a transfer text. What I eventually discovered was that one text field had a comma in the middle of it and when it was imported, it moved half the record over by one column and that changed my amount value for that record. That was the exact amount I was missing.
staring me in the face.....yes
Guilty as charged, but as you all know sometimes it's the most simple thing that we don't see right away!
I hope my explanation can help someone else who encounters this.