They're surely not covering 100% of the people, but there are very good chances that you will find a standard size fitting you well! This has clear advantages, for us as manufacturers, for our dealers, and for jumpers.
As manufacturer, we can organize the production better, allowing more favorable prices.
A dealer can stock standard sizes, with faster reassorting time, to serve better his own customers.
Jumpers can take advantage of a good quality product, at a reasonable price, with a short delivery time.
Men size chart
Women size chart