Why 1.705 MHz and FCC
Here's a pretty chart of all the radio spectrum allocations in the USA: https://www.nasa.gov/sites/default/files/spectrum_wall_chart_aug2011.jpg
They've been built up over many years in a fairly ad-hoc way, there's no maths behind it. Judging from that, I'd say they want to protect the radiolocation and mobile systems operating above 1.705MHz from interference, and don't care too much about the AM radio system below it.