Made 10 items with 80% chance of desynth success, failed 9 of them. I haven't been counting to see if this works out with the Law of Large Numbers, which would tell if the chance is accurate or not. I do know that the melding chances are at least fairly accurate. I calculated the costs to full meld my gear given materia prices and kept track of my fails. I used the fails to generate an actual cost and compared it against the estimated cost, and it was like, 13 million expected, 12 million spent. (I got lucky on one really hard meld which accounts for much of that million.) Of course, I've also failed a 45% meld 26 times in a row...

I think it's just that their random number generator isn't sufficiently random. It hits streaks where it is weighted to succeed or weighted to fail. In large numbers, it all evens out, but I think in short bursts, it can be overly negative/positive due to how it's coded. Obviously, you notice the negative streaks MUCH more than the positive streaks, like the time I pentamelded an accessory with zero failures.