This site describes the techniques used in canvas fingerprinting:
The technique is based on the fact that the same canvas image may be rendered differently in different computers. This happens for several reasons. At the image format level – web browsers uses different image processing engines, image export options, compression level, the final images may got different checksum even if they are pixel-identical. At the system level – operating systems have different fonts, they use different algorithms and settings for anti-aliasing and sub-pixel rendering.
So this sounds like it is fingerprinting the algorithms and config settings that your device uses to render a canvas.
It looks like wikipedia/Canvas_fingerprinting answers this question fairly directly, so I'll quote the relevant section in full (emphasis mine):
It sounds like if two devices are running the identical hardware and software, then they will produce the identical canvas fingerprint. So it seems like canvas fingerprinting is not a unique identifier by itself, but rather is part of a healthy diet of data that websites collect about you.