Several things make studying the reverse “Parion” dies challenging.
It is a simple geometric design, and the die cutters are cutting new dies to imitate the old. In this example, the design is very similar. The lines on the right are thicker, but this will happen as the die degrades. The die on the left could have had an additional line cut and then degraded to the line on the right. I don't believe that is what happened. I believe the die on the left deteriorated into a different shape, and then a new die was made copying the old.
The die is either soft or in use for a long time and slowly degrading. The die on the left degraded into the die on the right, and they kept using it.
The die cutters are tracing and enlarging die cracks. Here we see two impressions that look dissimilar, but I believe are from the same die, and closely related in time. The ancients seem to be widening either cracks or lines they traced themselves. I don't know if this technique would stop the crack from spreading.
Fakes. There are some casts out there. There are also some coins that seem to be unrelated to the main group. It is unclear if these are from short-lived dies, are ancient counterfeits, or are modern counterfeits.
I believe the reverse die I have been studying struck some of the earliest specimens. I could be completely wrong. It could merely be of poor style and soft. But it seems to be connected to the least-degraded obverses, and obverses of the most archaic style.