In an FIW PBEM, 9 units (~200 pwr) fully supplied in Fort Duquesne surrendered after a 30 day siege to a single cavalry unit (~20 pwr). I would've put a force outside the fort to swat this fly, but I didn't think it was a threat, and I was afraid my opponent might have a larger force on the way and didn't want to risk getting swatted in turn.
Judging from the manual, it seems he must've gotten lucky with siege roll and I must've gotten unlucky with mine, so that the siege resolution value was higher than the average discipline of my garrison, which consisted mostly of coureurs, braves and a few elements of colonial infantry under a leader with the fort defender trait. So my question is not so much, is this "working as designed" but rather, is this "working as desired"? I had mistakenly believed that the relative strengths of defending and besieging forces would come into play somehow, but it appears this is irrelevant, so now I am paranoid about any single unit siege of forts without supply wagons. It certainly doesn't inspire any confidence in the reputation of French military prowess =P