Nature of the Issue Faced :- Say you have 3 columns in a tablix/table on the report. There are 3 textboxes in each of these columns. Let us give them names as textbox1, textbox2 and textbox3.
Textbox3 should be textbox1 - textbox2. We are using following formula to achieve this :-

textbox3.Value = ReportItems!textbox1.Value -  ReportItems!textbox2.Value

textbox1 and textbox2 have decimal values rounded off to 3 decimal places (e.g.12.412). On the report we had set the format of the cell to be 'P1' (eg 12.4)

Now that we know the scenario, here is what the issue is - Everything works fine inside the cell of the report, however as soon as to export to excel the place where you have expression shows some arbitrary value like _385 -  _386

textbox1 has value of 76.0% and textbox2 has value of 74.1% in the above example!

Solution :- Changing the formula to Round(ReportItems!textbox1.Value,3) -  Round(ReportItems!textbox2.Value,3) rectified the issue. Cannot quite comprehend how it worked. Tried in SSRS 2008 and it worked fine!

I'll open this question to all of you who are reading this!

Thanks