Macrium Reflect image size differs

I’m trying to create an image of an SSD I need to use temporarily for another purpose. I’m using Macrium Reflect to create two partition images, one stored on an external HDD and the other on my FreeNAS server. The plan was to create and verify the two images, one after another, and then compare each image’s checksum.

The problem is that the images differ by a few KB each time I create them.

First of all I tried running Macrium on my Windows 10 install. As the images differed in size, I thought that the OS might have written to the disk between each image being taken. So I booted into my Macrium Reflect Rescue ISO and use it to create the images. Only I’m getting the same issue.

The partition images keep differing by a few KB.

What could be going wrong?

Is there a more reliable method of imaging a partition (either on Windows or preferably, *nix)?