The reason hydrogarden's hydroponic mix is low N is because it's designed to have the majority of the N provided by the calcium nitrate, like with most hydroponic mixes. These 2 part dry mixes like Jack's professional hydro, hydrogardens mix, and cropking's mix are all designed to be used with calcium nitrate as a part B. That is why they all have such high P and K. They contain the potassium nitrate and monopotasium phosphate, and magnesium sulfate.
Hydroponic systems without a medium should be supplied nitrogen in the form of nitrate, and must supply a soluble form of calcium. The only suitable form soluble calcium is calcium nitrate, which supplies the majority of both nitrogen and calcium in hydroponic mixes.
Soil and potting mixes fortified with lime, however, are meant to be fed with urea/ammonia based N, like jacks classic, miracle grow, shultz, peters, etc. These mixes do not provide any calcium as it's expected to be built into the medium.
With potting mixes, you can either omit limestone and use a hydroponic formula, or build with limestone and use a urea (lawn) formula.