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  • Pranav Gupta thinks this is interesting:

If you Shift-select points with the Direct Selection tool (instead of selecting them via dragging) before clicking an align button, points will align with and move toward the last point you select (called the key anchor point), regardless of which Horizontal or Vertical Align button you click.

From

Cover of Illustrator CS6: Visual QuickStart Guide

Note

Selecting anchor points with Direct Selection tool, does not not move the other key anchor point on aligning them vertically or horizontally