When you compare drills and battery packs, volts don't tell you the whole story. Most of the drills on the market use SubC NiCd cells, which come in several different sizes: 2/3 SubC, 4/5 SubC, SubC and 5/4 SubC. The cell diameter is the same and length is different. The capacity depends on dimensions and can be anywhere from 800 mAh for 2/3 SubC to 3500 mAh for 5/4 SubC.

No wonder Panasonic drill is.rated #1. It comes with 3.0 Ah battery, while Hitachi drill #4 comes with 1.4Ah battery.  They have15.6x3.0=46.8 Watt-hours vs. 18x1.4=25.2 Watt-hours.

DeWalt uses, I believe, 4/5 cells in 9.6V packs, while many others use larger SubC cells, which have 20-30% larger capacity. You can compare the height of two battery packs (just the thick part, not the "rod" that goes into handle). If you see the 1/4" or more difference, now you know why. So, if you are looking at two drills, try to find out the capacity figures for batteries expressed in Ah (Ampere-hours, Amp-hours) or mAh (miliAmp-hours)

Another variable is motor design, which can be more effective due to stronger magnets, etc., so it uses less electricity to produce the same amount of mechanical work, similar to MPG ratings in cars. However, it is not that easy to gauge it, so stick with the Ratings and adjust them somewhat for capacity.

Good luck!