It starts in Belgium with the Minimi... Before NATO standardization of 5.56x45mm.
1:9 was the original idea for the SS109 round, and this twist works just fine for it.
1:7 came about because it was needed to stabilize the L110 tracer in arctic conditions.
The Army decided that they weren't going to have a rifle that couldn't shoot both the ball and the tracer accurately under all conditions, so 1:9 never made it past discussion.