Magento: strict sequence for Users to choose options of a configurable product?
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It seems the order of options a Magento configurable product has, is forcing frontend-users to select options in exactly that sequence.

Example:

Shirts with 2 dropdowns:
  1.) sizes
  2.) colors.

A customer now _has_ to select the size, before the 2nd dropdown is even clickable or let them choose a color.

a) Why is that? 
b) And is there a way to change that so customers can start by selecting any options they would like to start with...?

Thanks for clarification