It is a variation in grind caused by thinning the heel after cutting to the plunge and establishing a discrete plunge line.
On most hand made knives the transition from the ricasso to the inferior portion of the edge is thick. As you may have noted, some makers cut a choil here or a "sharpening notch".
Since Andy does not like choils for reasons that are a matter of record, when this area is ground thin a slight recurve may be established.
If you sharpen an otherwise straight edge knife of this fashion to equal angles from tip to heel you will also grind in a recurve.
This even happens on machined blades.
It isn't wrong or right, but it is a trick to get that area sharp and straight. I would describe it as a part of the industry that is controlled for but only to a certain degree.
