At http://docs.gimp.org/en/gimp-images-out.html we can read :

// -----------------------------------------------------------
Subsampling Human eye is not sensitive in the same way all over color spectrum. Compression can use this to consider as identical slightly different colors. Three methods are available :
 • 2x2, 1x1, 1x1 : important compression; suits images with weak borders but tends to denature colors.
 • 1x1, 1x1, 1x1 : preserves borders and contrasting colors, but compression is less.
 • 1x1, 1x1, 1x1 (4:2:2) : between other both.
// -----------------------------------------------------------

The parameters mentioned at the beginning of the two last lines are identical. The three lines should probably begin with the following parameters :

2x2 1x1 1x1
1x1 1x1 1x1
2x1 1x1 1x1 (4:2:2)

Thanks to Gilles Caulier who noticed that error !