The difference actually lies in imperial and US measurements. The tablespoons per 5 gls actually bring it closer to the actual 0.1% I think if I remember correctly. Captk once ran the math for me. I'm no mathmetician,lol! Generally speaking, if we think about it, sometimes we have the disease known as CRS...

*cough* then its wise to remind folks to allow for the gravel and any ornaments displaceing water volume also. A salt test kit is also a big help in keeping it as close to or spot on the doller!
Accordingly, amounts have been used up to the 0.5% range without problems and sometimes is called for when things (noted in ich & Costia) are showing resistence to the lower amounts. So even at 1 tsp per gl, we have some safety (salt creep)allowance......