As far as I understand, Telerik does not use the stored value, is produces a RadExpression and computes the value itself. This works sometimes, not always.
I figured a little for the conditions, for me it failed on cross-workbook references.
Imagine you have a workbook (xlsx-File), put the numbers 1,2,3 in Cells A1 to A3.
Put the Sum(A1:A3) in any cell of the workbook, even on other sheets, GetResultValueAsString will return "6".
But put the Sum in another workbook:
=SUM('F:\Telerik\[Mappe1.xlsx]Tabelle 1'!A1:'F:\Telerik\[Mappe1.xlsx]Tabelle 1'!A3)
will result in GetResultValueAsString returning "=SUM('Tabelle 1'!A1:'Tabelle 1'!A3)"
and considering the cell as a Text cell.
I guess this feature of excel is rather rarely used, and not supported by your product.
The  seems to be an internal index to an unresolved reference.
But beside a feature request, to support this type of reference and process the formula correctly,
it would be another feature request (or may be it's already somewhere), to access the stored value.
In the xslx File the cell looks like this:
<c r="A1"><f>SUM('Tabelle 1'!A1:'Tabelle 1'!A3)</f><v>6</v></c>
the "6" is there in the <v>-Tag, but I can't access it, and the <f>-Tag is considered as Text.
Excel itself stores the value, to display something useful, even if the reference is broken, f.e. if you send the single workbook to someone by e-mail.