They are pretty inconsistent because SUM(C2:C6) does not yield the #VALUE error value, but simply ignores the non-numeric cells, i.e. adds them as being 0.
The real problem for fpspreadsheet I think is that, even when C2+C3+C4+C5+C6 were allowed to ignore the non-numeric cells, the sum is not correct: cell C7 should show 14, but not 12! I'll have a look.