Those transmissions originally came from the factory filled with Case HyTran hydraulic oil. At some point- roughly 2000-2001, Case and/or ZF changed their mind, and decided that these machines (621,721,821, B, C) should use 15W40 in the transmissions. Not sure what the reasoning was, but I did swap the fluid in several of them. Seeings how the viscosity of 10w30 will generally fall somewhere between that of hydraulic oil and 15w40... I wouldn't worry about it too much.
I do that know some of the bigger Allison transmissions can use either DexIII or 15W40, depending on temperature range and application. I've been told (haven't seen it in writing, though) that the transmissions produce less heat when running on 15w40 due to less slippage in the torque converter.