"Coffin corner" is where the high and low speed constraints come close together, ultimately leading to an altitude where at any except the perfect speed, you are either too slow or too fast. It's as much a problem with being slow as being fast.
So at some slightly lower altitude, the "point" speed becomes a range - with a lower and upper bound, below or above which you are again either too slow or too fast. And somewhere in the middle of that range of usable speeds is where you'd get the best in terms of margins.
So if, for instance, 0.79 was the lower bound at the altitude you were looking at, and 0.82 were the upper bound, it's entirely possible that 0.81 is a little better than 0.80. What's happening is that at 0.80, the low speed limits are predominating, and it's better to go a little faster and open up the lower speed constraint, as the upper speed one hasn't started to "bite" yet.