Ok that wipes out my first theory of high mash temperature. (It may still have some effect on overall results mind you, but not between fermenters)
However theory #2 and let me preface this by saying I know bugger all but this might be it.
Reading the spec sheets on Nottingham Yeast it has a high attenuation. Somewhere between 80 to 85% whereas S-04 is a more standard yeast with an attenuation of around 70 to 75%
So I'm thinking your Nottingham worked harder, converted more of the available sugars thus a lower FG. The S-04 has still done its job and is probably fully fermented but only around the 70 - 75% mark. From what I read this will give you a slightly sweeter beer than the Nottingham will. Obviously you are doing this as a bit of an experiment to see which style you prefer.
So I am learning as I type this, but there's a few formulas around to work out attenuation. Or just plug the different values into Biabacus in Section H (Attenuation) and see what it spits out in Section A. (Estimated FG)
But being the spec sheets of each yeast show at least a 10%. difference I think that may be your answer and your ferment in both is done and dusted.
(Maybe)
Below an article from BYO that may explain it better
Attenuation:
Attenuation refers to how completely the yeast can ferment wort. If your wort’s starting gravity is 1.048 and its final gravity is 1.012, then the attenuation is calculated as (48-12)/48 = 0.75, or 75%. This value is about average. Some yeast attenuate less — in the 70-73% range — resulting in a sweeter, fuller-bodied beer. Wyeast 1968, White Labs WLP002 and Danstar Windsor are examples of low to moderate attenuating yeast. Other yeasts attenuate more — as much as 80-85% — resulting in a drier, lighter-bodied beer. Wyeast 1010, Danstar Nottingham and White Labs WLP007 are examples of highly-attenuating yeast strains.
