It's a normalization factor. The "watts per gallon" guideline is set up for T-12 bulbs. Bulbs other than T-12's have different efficiencies as far as trasnmitted the light to the water/plants. CFL's are less efficient. Meaning, if you compared a 26w T-12(it doesn't exisit, but for the sake of argument let's pretend it does) and a 26w CFL, the CFL would only put 86% as much power into the aquarium as the T-12. So, in order to be able to apply the watts per gallon guideline to CFL bulbs, we have to multiply by .86 to tell you what it would be in t-12 terms.
And, for your case, t-5's are better at transfering power from the lights to the plants, so you would increase by multiplying by 1.37. This would tell you your watts per gallon in the standard t12 convention.
And, for your case, t-5's are better at transfering power from the lights to the plants, so you would increase by multiplying by 1.37. This would tell you your watts per gallon in the standard t12 convention.
