The Files field of a FileDataStore is a cell array of character vectors that holds file names. When you cell2mat() that, you get a character vector or character array. Character vectors and character arrays are treated as numeric when you do numeric operations -- for example,
looks up the Unicode code position for the letter 'A' (which is 65), and adds 1 to that code (getting 66 -- which happens to be the code position for the letter 'B')
Thus, you are doing mathematics on the characters of file names, not on the contents of the files.