I saw that using `abs -t' will extract all package source directories to $ABSROOT instead of $ABSROOT/$repo/. Then I found the structure of Parabola's abs tarball is different from Arch's.

Arch:

tar tvf core.abs.tar.gz | head

drwxr-sr-x root/root 0 2015-12-29 07:07 core/
drwxr-sr-x root/root 0 2015-12-29 07:06 core/nilfs-utils/
rw-r--r- root/root 814 2015-12-29 07:06 core/nilfs-utils/PKGBUILD
drwxr-sr-x root/root 0 2015-12-29 07:07 core/s-nail/
rw-r--r- root/root 1563 2015-12-29 07:07 core/s-nail/PKGBUILD
drwxr-sr-x root/root 0 2015-12-29 07:06 core/nano/
rw-r--r- root/root 413 2015-12-29 07:06 core/nano/nano.install
rw-r--r- root/root 994 2015-12-29 07:06 core/nano/PKGBUILD
drwxr-sr-x root/root 0 2015-12-29 07:06 core/linux-api-headers/
rw-r--r- root/root 1462 2015-12-29 07:06 core/linux-api-headers/PKGBUILD

Parabola:

tar tvf core.abs.tar.gz.1 | head

drwxr-xr-x root/root 0 2015-12-29 07:05 acl/
rw-r--r- root/root 903 2015-12-29 07:05 acl/PKGBUILD
drwxr-xr-x root/root 0 2015-12-29 07:05 attr/
rw-r--r- root/root 1092 2015-12-29 07:05 attr/PKGBUILD
drwxr-xr-x root/root 0 2015-12-29 07:05 bash/
rw-r--r- root/root 5569 2015-12-29 07:05 bash/PKGBUILD
rw-r--r- root/root 400 2015-12-29 07:05 bash/bash.install
rw-r--r- root/root 21 2015-12-29 07:05 bash/dot.bash_logout
rw-r--r- root/root 57 2015-12-29 07:05 bash/dot.bash_profile
rw-r--r- root/root 141 2015-12-29 07:05 bash/dot.bashrc