In short, jamgrass is a freer and more expansive version of traditional bluegrass. But it helps to view it in terms of how all music, like language itself, evolves.
Music is a language. And as language evolves, so too does music.
Some would say the sensory and atonality of of Debussy (said to be influenced by a literary style in France in the late 19th century known as Symbolism) somehow was the progenitor to American-born jazz. And jazz, known for its improvisational jams, could well be what evolved traditional bluegrass music to jamgrass sometime in the latter half of the 20th century.