Edit: Oh wow, and adding a third wrinkle here:

• Lemmy.world says 9.25K upvotes
• I was talking below about sh.itjust.works showing 7.86K
• Your home instance Lemmy.ca does not even have this post at all - it has posts from that exact community and from around that time though, so perhaps your admin blocked the OP user account prior to that post being submitted?

It is normal to see such differences, although in this case I am not sure what instance sh.itjust.works may have been defederated from at the time that Lemmy.world was not - perhaps it was hexbear.net prior to Lemmy.world defederating from it? Or a LOT of user accounts on lemmy.world or elsewhere in the Threadiverse but blocked on sh.itjust.works?

It also looks like Lemmy.world's sort by old is broken somehow.

You can see all the individual upvotes and downvotes on lemvotes, but that's a lot for a human to read through.

I guess the OP submitted the post from their home instance, but then the community it was submitted to is located on Lemmy.world so it may be the "official source" in such a case? e.g. if someone were to use the Lemmy instance block feature on sh.itjust.works then this post would still come through, while if someone instance blocked Lemmy.world then it would not, despite the OP's instance being otherwise.

That's weird to be such a noticeable difference between the two!! (Edit: now three)