High Sierra 10.13.3 Supplemental Update

I use iMac 27 inch (18" early) which using fusion drive.

(current os version is 10.13.3)

It's drive format is mac OS Extended.

I know "The High Sierra 10.13.3 Supplemental Update" support only for APFS Format, not OS Extended.

(I realize this fact from install update on Apple website, the install program warn this fact and update failed.)

But App Store asks me to install the update without a break.

If I install the update, my computer install the update and restarted. Then there are no more updates.

However when i shutdown my computers and restart, then there are the update which i install just before.

My questions is,

Why App Store asks me to install "10.13.3 supplemental update" even if my format is macOS Extended?