The Mk1 came with a 95bhp 1.8 8v (RT, Baccara) and a 110bhp 1.8 8v (RSi).
The 137bhp 1.8 twin cam 16v (16V) was a different design, from the high performance engine series.
Newer cars have lower insurance groups because of commonality of parts (e.g. a Golf is also a Leon, Octavia, TT, A3) and because they are, on the whole, less easy to steal.
Insurance groupings given when new mean very little to older cars, for reasons including the likelihood of repair going down (and therefore cost).