You're right. "Bars" are only useful for comparing signal strength with other "bar" measurements on the SAME MODEL. Other than this, bars are a useless means of measuring anything. What really matters is signal strength, measured in "db".
Check out Wifi Analyzer in the Play Store. It will show you your current signal strength in db, how crowded each channel is, and recommend a better channel if one exists. Keep in mind that the signal strength shown will be in the "less than zero" range, and that a signal strength of "-70" is better than a signal strength of "-90" (the channel graph appropriately reflects this).
Also, an odd issue I had with a Droid X was that extended battery was being used with the standard battery cover - causing additional pressure on some internal components. It was eventually determined that this alone was causing a 30db drop in signal strength.