Its simply treating them as text. + is considered a valid concatenation operator in Access when the fields in question are text. Using VAL() will convert these text fields to their numeric value and allow addition.
=val(nz([txtBox1Name],0)) + val(nz([txtBox2Name])) + val(nz([txtBox3Name])) + val(nz([txtBox4Name])) + val(nz([txtBox5Name]))
J