Plus these devices are using different chipsets, so I wouldn't be surprised if that was a factor. Along with differences in manufacturing.
To get a better idea, and I am not recommending this because of DLM (please, don't do this if you don't want to mess things up), run the same test mutliple times - you could then average out the values for each device and they may sit closer together (except for the Plusnet Hub One, I'd imagine, since it is Lantiq).
All the modems are within about 1 dB of each other, so nothing to worry about here.