FreeOrion ship production variables

I'm playing FreeOrion (a free 4x game based on Master of Orion), and when I put a ship in the production queue, there are two drop-downs at the top-left which both affect the total number of ships that get built. But, I don't understand the difference between the two options:

You can see that I can choose 1, 2,3,4,5,10,20,50, or 99; and choosing 2 ends up creating two ships. But, the other option right beside it also changes the number of ships built, but it seems like choosing that one is slower.

My Question: What are the the two options changing, besides just # of ships produced?