Because t5's put out a lot of light in wavelengths not used by the plants, which means you waste a lot of that energy. Or to pu it another way:
Let's say that only 20% of the light emitted by t5's are absorbed, this means that the effective lumens pr. Watt is just 18.5, whereas the LED lights might be 95% efficient - meaning 76 lumens pr. Watt in your example above - that's a ratio of app. 4:1!
Looking at it this way they are far superior to flourescents, it's just a shame they are so expensive..
I hope I was clear, otherwise just ask
Christian