What osm2pgsql and osmosis exactly do when import data into postgis database?

I have set up the rails port on ubuntu14.04, and then I build my local tile server under the instruction of "Manually building a tile server (14.04)". And I have changed the OSM map to my local tile server. The nominatim is still use the http://nominatim.openstreetmap.org/

I have dowmloaded the beijing_china.osm.pbf data file from the Metro Extracts, then i use osm2pgsql to import data into the tile server database,such as "osm2pgsql --slim -d gis -C 1600 --number-processes 3 /usr/local/share/maps/planet/beijing_china.osm.pbf", and i use osmosis to import data into the rails port database,such as "./osmosis-latest/bin/osmosis --read-pbf /home/osm/planet/beijing_china.osm.pbf --write-apidb host="localhost" database="openstreetmap" user="osm" password="buaanlp" validateSchemaVersion="no"". What confuse me is that the table structure in this two database is quite different, then how could it use the same data file to import data?

The table structure in the rails port database and in the tile server database can be showed as blow.

rails port database

tile server database