It was at a crossing over an effectively-bottomless pit. It made narrative sense in context; and from a campaign design perspective the proximity to the pit was to constrain player options by preventing us from avoiding the NPC altogether. Yes, the size restrictions and contested checks were all complied with. (I mean, I don't know the specifics of the NPC's end of the check, but knowing the DM and the circumstances overall, the likeliest inference by far is that the stats weren't adequate to make up for a really poor roll. Having actually been there, I have no basis to assume otherwise, as you seem to.)

The DM clearly neither expected the player choices, nor the outcome of the dice. (From a metaplay perspective, I'm quite certain that this was not meant to be a combat encounter at all.) The outcome required a lucky roll by the player, and an unlucky roll by the NPC, and was no doubt improbable - but the dice are fickle.