Since August the increase in difficulty has been quite stable around the 25-30% mark , with a spike to 40 in late october but followed
by under 20% for the next 2 adjustments.
If you didn't take this into account , then you wasted your time with a wrong profit calculator or you get the numbers wrong.
Spikes aren't predictable, and they're not nice to get in your foot. Treading lightly doesn't help much in the mining game, because you're bound to find one eventually...which was the point of that post; there's nothing predictable about difficulty and calculators offer false reassurance.