Volume keyword in Gaussian

Dear CCLers,
 I would like to ask about Volume-keyword. I am interested about the volume
 of molecule. But for example with following six input files I get
 different molar volumes:
 1)
 Mem=150MW
 #p rhf/3-21g* Density=Current scf=tight volume test
  Box volume =  982.544 fraction occupied=0.116
  Integrated density= 5.0945197336220325D+00 error=-4.9054802663779675D+00
  Molar volume =  114.134 bohr**3/mol ( 10.185 cm**3/mol)
  Recommended a0 for SCRF calculation =  2.25 angstrom (  4.26 bohr)
 2)
 #p rhf/3-21g* Density=Current scf=tight volume test
  Box volume =  982.544 fraction occupied=0.141
  Integrated density= 7.8469300328767240D+00 error=-2.1530699671232760D+00
  Molar volume =  138.946 bohr**3/mol ( 12.399 cm**3/mol)
  Recommended a0 for SCRF calculation =  2.37 angstrom (  4.48 bohr)
 3)
 #p rhf/3-21g* scf=tight volume test
  Box volume =  982.544 fraction occupied=0.162
  Integrated density= 8.7064056753546222D+00 error=-1.2935943246453778D+00
  Molar volume =  158.795 bohr**3/mol ( 14.171 cm**3/mol)
  Recommended a0 for SCRF calculation =  2.46 angstrom (  4.64 bohr)
 4)
 %Mem=100MW
 #p rhf/3-21g* scf=tight volume test
  Box volume =  982.544 fraction occupied=0.152
  Integrated density= 2.7978807764929656D+00 error=-7.2021192235070348D+00
  Molar volume =  148.870 bohr**3/mol ( 13.285 cm**3/mol)
  Recommended a0 for SCRF calculation =  2.41 angstrom (  4.56 bohr)
 5)
 #p rhf/3-21g* opt scf=tight volume test
  Box volume =  982.544 fraction occupied=0.146
  Integrated density= 7.0641561577918202D+00 error=-2.9358438422081798D+00
  Molar volume =  143.908 bohr**3/mol ( 12.842 cm**3/mol)
  Recommended a0 for SCRF calculation =  2.39 angstrom (  4.52 bohr)
 6)
 %Mem=100MW
 #p rhf/3-21g* opt scf=tight volume test
  Box volume =  982.544 fraction occupied=0.126
  Integrated density= 8.0829021488536519D+00 error=-1.9170978511463481D+00
  Molar volume =  124.059 bohr**3/mol ( 11.071 cm**3/mol)
  Recommended a0 for SCRF calculation =  2.30 angstrom (  4.35 bohr)
 How I should use Volume keyword to get the right volume of molecule ?
 Thanks in advance!
 Yours,
 Maija Lahtela-Kakkonen
 ***************************************
 Maija Lahtela-Kakkonen
 Researcher
 CSC-Center for Scientific Computing
 Tekniikantie 15 a D
 P.O.Box 405
 FIN-02101 ESPOO FINLAND
 TEL 358-9-4572079
 FAX 358-9-4572302
 E-MAIL mlahtela' at \`csc.fi
 ***************************************