Why does `ls` in Linux and macOS show different owners (uid) for the same file?

I copied some files to HFS+, using macOS, ensuring that it was copied exactly. On macOS these copied files have 501 as owner according to ls -han. I then plug in the HFS+ usb stick into Ubuntu, and there the files have 1000 as owner according to ls -han. Why? I then tried copying one of the 501 owned files in Ubuntu (to the same HFS+ volume), ensuring that it was copied exactly using cp -a. Now macOS ls sees the new file as owned by user 1000... Really? I don't understand — what was the point of using cp with the -a option if it doesn't even preserve the owner's user id? What did I miss? **Update:** To clarify, I think my confusion here stems from that — in my mind — HFS supports Unix file permissions natively and should "just work" with them. --- I recently learned that cps preserve=timestamps does not, in fact, preserve time stamps (creation dates are reset). Am I now to believe that its preserve=ownership does not preserve ownership?