AFAIR this is the implant angle (off perpendicular onto the wafer) for <100> oriented silicon wafers.
It has nothing in common with the Well Proximity Effect.
See here an explanation from the Allen/Holberg book, chap. 2 :
The average depth of penetration varies from 0.1 to 0.6 μm depending on the velocity and angle at which the ions strike the silicon wafer. The path of each ion depends upon the collisions it experiences. Therefore, ions are typically implanted off-axis from the wafer so that they will experience collisions with lattice atoms thus avoiding undesirable channeling of ions deep into the silicon.