2 Waterman Havana Brown samples. One is in cartridge form the other inside of a converter. Both are in Waterman Pens.

Why is the cartridge ink a darker brown than the bottled ink? Is this the norm? Why?

Cartridges tend to be a little different in color. If the cartridges are old they actually experience evaporation and the ink concentrates.

In addition to what was previously said, when you filled the pen, there could have been a little bit of water in the converter. Also, if the pens have different flow (i.e. wetter or dryer) that would affect the way the ink looks.