Comment 4 for bug 1905319

And taking a look at the internals of your file, it's because it is using
an obsolete form of markup for the images (VML). calibre doesn't really
support conversion of this markup, defaulting to just placing the image
as a block on its own, which is not styled in anyway, therefore not
centered. If you simply delete the images and re-insert them in word
you should be fine, as Word will use modern markup automatically.

That said, I will change calibre in the next release to not put such
images in a block on their own, which will fix your issue, since they
will inherit the centering from the parapgraph they are in.