Docker re-pushes pre-existing images to private registry

Docker Engine
1

I pulled an image from my private registry (running registry:2 from Hub), then immediately pushed it again. I expected to see “Layer already exists” for all layers, but one of the layers re-pushed:

$ docker pull my-private-repo.redacted.com/my-image:1.0.0
1.0.0: Pulling from my-image
Digest: sha256:4d61fd5419960151dc37a3fa4ba0e0a8236de2f816cbaf7445c550938e1d1b1b
Status: Image is up to date for my-private-repo.redacted.com/my-image:1.0.0

$ docker push my-private-repo.redacted.com/my-image:1.0.0
The push refers to a repository [my-private-repo.redacted.com/my-image]
72766d235de7: Layer already exists
694887710c2a: Layer already exists
8b53afdf0fad: Layer already exists
5404de47124a: Layer already exists
8c86c772e7b9: Pushing [==>                                                ]  15.82MB/270.4MB
5a5b72363265: Layer already exists
c7b3a0504d15: Layer already exists
9382c870b5a0: Layer already exists

I’m the only user of the repo.

The image in question was originally built and pushed to the registry from a Linux machine:

Client:
 Version:      17.06.2-ce
 API version:  1.30
 Go version:   go1.8.3
 Git commit:   cec0b72
 Built:        Tue Sep  5 20:00:17 2017
 OS/Arch:      linux/amd64

Server:
 Version:      17.06.2-ce
 API version:  1.30 (minimum version 1.12)
 Go version:   go1.8.3
 Git commit:   cec0b72
 Built:        Tue Sep  5 19:59:11 2017
 OS/Arch:      linux/amd64
 Experimental: false

I then pulled and pushed the image from a Mac

Client:
 Version:      17.06.0-ce
 API version:  1.30
 Go version:   go1.8.3
 Git commit:   02c1d87
 Built:        Fri Jun 23 21:31:53 2017
 OS/Arch:      darwin/amd64

Server:
 Version:      17.06.0-ce
 API version:  1.30 (minimum version 1.12)
 Go version:   go1.8.3
 Git commit:   02c1d87
 Built:        Fri Jun 23 21:51:55 2017
 OS/Arch:      linux/amd64
 Experimental: true

Why does Docker upload pre-existing images? The image layers are the same.

Thanks,
Rich