Sometimes it is the small things that make the final impact on the bigger things. This is especially true in soybean yields. Basic components of soybean yields are:
Plants per acre X seeds per plant
Seed Size
Seeds per acre component is set around pod set. (R3-R4) This means during the later stages of a soybean's life cycle the seed size is what impacts your yield. Water is typically the critical play at this stage. The fertility should be right, weeds under control by this time, and hopefully disease is not an issue. Other weather factors such as sunny vs cloudy, high temps vs low temps, humidity and wind speed can affect the stress of the plant.
Many are not fully aware of the degree of yield impact that occurs from the size of the harvested seed. Perhaps, this is why soybeans can be estimated on yield with great care, but the final yield result can make that estimate look like a wild shot in the dark.
I am including a chart screen shot from the University of Nebraska-Lincoln to illustrate the yield swing from seed size change. I apologize for it being somewhat grainy and hard to read.
As final side note, how do you know if the soybean has stopped filling the pods and has reached R7 or physiological maturity? At the soybeans equivalent to corn black layer you can slit the soybean pod at the suture line. Then, note if the interior membrane clings tightly to the seed coat. If it does the plant is still feeding carbon and nitrogen metabolites into the grain. However if the membrane stays with the pod wall and does not cling to the seed coat, the seed is no longer receiving anything from the mother plant. The final yield component of seed size is now fixed.
References:
University of Nebraska-Lincoln
Dr Jim Specht UNL Professor of Agronomy & Horticulture
