What is the difference between solving in one step and solving separately

Hi,

I have built a modeling including two modules, one is laminar flow and the other is transport of diluted species. Strangely, I found there is a little difference for the results between solving these two modules together using one step and separately solving the two modules using two linked steps. Could you explain why this difference happened.

If so, when can I separately solve modeling? And when I need to solve them together? Thank you.