My IBM lenovo thinkpad battery extension lasts only 2 hours instead of 4 hours described in spec

Spec of my thinkpad is,

ThinkPad X61s : Intel Core2 Duo processor L7500, 軽量12.1"(XGA), 指紋,
1GBx2, 160GB, MIMO(2x2), BT, 4セル拡張容量Battery
windows vista

as described above, i think i am using this battery;
http://www-06.ibm.com/jp/pc/option/obi/nob07/40y7001/40y7001a.shtml
when using without ac, i tend to close programs and even processes out of use, but the result is for only 2 hours that is half of what spec says.
little bit disgusting.