The low self discharge (LSD) batteries are a different internal structure - they aren't just good quality NiMH.
You get much lower self discharge but at the cost of 20-25% less capacity, however they keep the same output voltage for longer so if your device has a high 'low battery' voltage they may last longer.

Their initial output voltage isn't much different from NiMH - your measurement of 1.5V is faulty because the extreme high input impedance on a DVM means you are mostly measuring stray charge on the terminals, run them for a minute and it will measure 1.2V or so.

If you're shopping for them the original Duracells marked 'made in Japan' are repackaged Sanyo Eneloops.

ps. my earlier post it was 4xAA for $10 in Dell's sale - I forgot I put the other two in my GPS