Wondering about ballot counting

Yesterday was an election day in France (next Sunday will be too) and as always, I participated in the counting process of the ballot after the election closure.
After reading this article (and it is not the first I'm reading about the various problem with ballot counting in the US), I'm wondering why it seems so difficult to get a correct process there ?
Currently, in France, election process is quite simple : there are paper ballot for each candidate (or candidate list). Each elector should take 1 paper ballot per list, do his choice in the insulator and place the envelop containing his vote in the ballot box.
Once the election is off, voters who volunteered during the day are doing the counting process:
they are grouped by 4 person by table, then receive 100 ballots : one person open the envelope, another one get it, checks it is ok and announce the vote. The two other person have accounting the vote separatly on paper list and at the end of the 100 ballots, both lists are compared to check for errors.
It might be as fast or high-tech as electronic voting (it tooks around 1h to check 167 ballots for one table and there were 3 teams doing the same work at my election office) but I think it is quite safe and not as disturbing as the process error I can read in the Wired article (miscalibration of optical scanner, paper jam and so on).
So, I wonder, why ballot counting seems so problematic in the US (either the punch card, optical or electronic voting) ? Isn't paper ballot not enough ?