While creating Reporting Services (SSRS) reports, I usually use Format Functions around values to make the report look good (mostly for Number with decimal and comma). By doing that way while export to Excel we may lose the ability to sum the values, because everything is exported as text.
There is a right way to make it work. Remove all the Format function like Format(‘Field.xxxx’, "N2"). On all your number fields, highlight them, then in the properties area, there is a “Format” property. Set Format Code in that to N2 (N for Number, 2 for the number of digits after the decimal, so if you wanted zero, it would be N0). Again for Percentages & currency its the same way P2 or P0.