It is rather simple - here is my crack at supplying a simple answer:

1. Gearing is the ratio of the number of teeth of the front chainring and the rear cogs:

Ratio=NT(front)/NT(rear); the Vitesse has 9 rear cogs so has 9 of those ratios, hence a gearing "range" of 31"- 96". (Wheel diameter in inches is multiplied with simple cog teeth ratios to yield inches.)

2. The TT has higher gears because it has an internal geared hub, which supplies additional step-up and step-down gears compared to the Vitesse; so

Ratio1 = 1.33*NT(front)/NT(rear);
Ratio2 = 1.0*NT(front)/NT(rear);
Ratio3 = 0.75*NT(front)/NT(rear)

Each of these Ratios come in 9 values, hence 27 gears.