What does it mean that uranium always refills from space 16?
The rulebook is confusingly worded. The "Exception" is calling out a graphical peculiarity, not a rules difference.

The resupply for Uranium works the same as for every other resource: you take the specified number of units and place them on the most expensive slots available. This exception is calling your notice to the fact that there are four extra Uranium slots that do not share one of the large square blocks with Coal/Oil/Garbage, and that once those are full, there is only one slot for Uranium in each of those square blocks.

You can see that this is the case in the example immediately after your quote on p. 6 of the rulebook. The market begins with 2 Uranium on 14 and 16. The table calls for 2 Uranium to be placed, and those are placed on the 12 and 10.