I'm not certain, but based on the description by the author, this solver is sometimes doing probabilistic guessing. You're more likely to make a mistake early when you do this -- there are fewer chances to guess incorrectly near the end of the game as inferences grow stronger, and more chances in the beginning and middle to guess wrong which will then snowball into a more wrong answer. So statistically, it makes sense that failures tend to be large.

I thought it was interesting that the "hard" category seemed to have more wrong answers than the 2 harder categories. Maybe there just aren't enough samples, but in the data shown in the table, it seems like a big difference.