Lake Manasarovar v.s. Lake Rakshastal: fresh-water v.s. salt-water
You pretty much answered the question yourself.  Lake Rakshastal, along with most of the lakes along the Tibetan plateau is, indeed an endorheic lake.  The important meaning of that term is that there is no outflow from that basin.  Water evaporates, but as with any evaporative process, the minerals in the water stay behind and become more concentrated.  Since Lake Manasrovar flows into Lake Rakshastal, the water that leaves takes the minerals in contains with it, leading to a much lesser concentration left behind as new water flows in, and, most importantly, out of that lake.