why 6 not upgraded in apt dist-upgrade?

# LC_ALL=C apt dist-upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following packages have been kept back:
  gir1.2-lokdocview-0.1 libreoffice-base libreoffice-gtk3 libreoffice-math libreoffice-report-builder-bin libreoffice-sdbc-postgresql
0 upgraded, 0 newly installed, 0 to remove and 6 not upgraded.