I'm very much a visual person as well. I believe the load width in millimetres across the top is the centres at which the bearers are placed. So if we take the first set of 900mm centred bearers, your 150x50 at a single span can span 3300mm. If you increase the width between your bearers to 1200mm, your bearers can now only span 3000mm, and so on. The greater the distance between bearers, the less they can span as they'll be individually carrying more weight.
Let me mention one of our knowledgeable members, @ProjectPete, to see if he can confirm for us.