I'm going to take a stab at this one- a real stab in the dark.

Hydro is different because there is such an excess of water and chemical components remain dissociated into their respective ions. Plus the water is supposed to be oxygenated by a bubble stone. RedOx chemistry is different.

In soil, especially cold, wet, nonporous soil, since there is not that water matrix plus there is too little oxygen, chemical components behave differently. That is why when you see pH problems in your soil-look at the FAQ to confirm this- most nutrient lockouts are exacerbated by poorly-draining soil.

I'm not familiar emough with the behavior of fertilizer component mixtures in water to really explain this properly.