Why does GNU tar --format=pax produce ustar archives?

I'm using GNU tar (v1.29) to create an archive and xxd to inspect it like this:

(
    cd "$(mktemp -d)"
    touch -d@0 1.txt 2.txt 3.txt
    tar \
        --format=pax \
        --numeric-owner \
        --owner=0 \
        --group=0 \
        --pax-option="exthdr.name=%d/PaxHeaders.0/%f,delete=atime,delete=ctime" \
        --mtime=@0 \
        -cf ../123.tar \
        1.txt 2.txt 3.txt
    xxd ../123.tar | grep -C1 ustar
    rm -rf $PWD
)

Which gives me this:

000000f0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000100: 0075 7374 6172 0030 3000 0000 0000 0000  .ustar.00.......
00000110: 0000 0000 0000 0000 0000 0000 0000 0000  ................
--
000002f0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000300: 0075 7374 6172 0030 3000 0000 0000 0000  .ustar.00.......
00000310: 0000 0000 0000 0000 0000 0000 0000 0000  ................
--
000004f0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000500: 0075 7374 6172 0030 3000 0000 0000 0000  .ustar.00.......
00000510: 0000 0000 0000 0000 0000 0000 0000 0000  ................

Why does tar --format=pax produce an archive with ustar in it?

Solution

pax Interchange Format:

A pax archive tape or file produced in the -x pax format shall contain a series of blocks. The physical layout of the archive shall be identical to the ustar format described in ustar Interchange Format.

"ustar" followed by 1 zero/NUL byte is the value of the magic field indicating the type of the archive:

The magic field is the specification that this archive was output in this archive format. If this field contains ustar (the five characters from the ISO/IEC 646:1991 standard IRV shown followed by NUL), …

Of course, that's only for any conforming pax utility, but I'd expect pax format archives created by GNU tar to create archives in the same way as a conforming pax implementation.