These angles are best construed to mean the angles observed (measured) for a given system, and not an historically observed range in values (e.g., as in the  range of the original Bürgi–Dunitz aminoketones), or an idealized value computed for a particular system (such as the  = 0° for hydride addition to formaldehyde). That is, the  and  angles of the hydride-formadehyde system have one pair of values, while the angles observed for other systems—combinations of nucelophile and electrophile, in combination with catalyst and other variables that define the experimental condition, including whether the reaction is in solutio or otherwise—are fully expected (and are reported) to vary, at least somewhat, from the theoretical, symmetric hydride-formaldehyde case.
A stated convention for  is that it is positive (>0°) when it deviates in direction:
* away from the larger substituent attached to the electrophilic center, 
* away from the more electron-rich substituent (where these two and other factors can be in a complex competition, see below); 
hence, as noted,  for reaction of a simple nucleophile with a symmetrically substituted carbonyl (R = R, or other symmetric planar electrophile) is expected to be 0° in vacuo or in solutio', e.g., as in the case of the computed and experimental addition of hydride (H) to formaldehyde (HC=O).