It's to do with the sensitivity of the eye to the colour spectrum emitted by the bulbs. A light meter will probably show that tungsten and low-energy bulbs are emitting around the same amount of light, but your eyes don't think so. I mentioned this to a supplier, who said 'we're still working on that'. They obviously haven't got it right yet!
I try to use the next highest equivalent wattage to a tungsten bulb. Thankfully there are now 125w low energy bulbs to replace my 100w tungsten ones.