Purification of the Hünig Base
Ref: Purification of Laboratory Chemicals, 7 ed. 

In the first case, your labs 'common procedure' isn't the 'common procedure' that most people tend to follow. Generally, it should be carried out as follows


Distill the DIPEA from ninhydrin at atmospheric pressure. The ninhydrin reacts with any primary/secondary amines, removing these impurities from the DIPEA. This ninhydrin step liberates water, as it works by condensing the primary/secondary amines.  Since the boiling point of DIPEA isn't that distinct from that of water, people tend not to try fractionally distilling the two away from one another. The DIPEA that comes across in this first distillation is free of primary/secondary amines, but contaminated with up to ~1% water (which is far too much to get away with using in moisture sensitive chemistry). 




Ref: Chemistry LibreText 


Re-distil the 'wet' DIPEA from KOH, again at atmospheric pressure, which removes water and provides clean and dry base. 


The reason why calcium hydride is often not used is because it is thought to promote the degradation of DIPEA by cleavage of the isopropyl groups (they can eliminate to give stable products. Certainly, distillation from calcium hydride is usually fine, but the DIPEA shouldn't be stored over it for prolonged periods.