The rai·son d'ê·tre.  ABC/R's lower paddle is angled and protrudes further laterally, easily separating itself from the ambidextrous magazine catch's release paddle.  It enables the user to identify and differentiate these two different components by feel, without a visual.

ABC/R's top paddle's upper half is angled at 10 degree, and lower half is angled at 5 degree, to make accessing the bolt release easier when the bolt carrier is locked back.  Our good friend Nate Schultz (known on various social media as NSZ85) has influenced the ABC/R's top paddle design.

Lightening cuts on the ABC/R, with its enlarged upper paddle, help keep the weight (0.445 oz) close to that of a factory bolt catch (0.401 oz)

The horizontal serrations of the top paddle continue further below, and terminate at the notch.  The additional serrations here help provide more traction on the ABC/R's top paddle.

The notch immediately above the ABC/R's lower paddle serves an important function.  It provides tactile feedback to the user as to the location of his finger on the bolt catch, and differentiates the two control surfaces on the ABC/R (upper paddle and lower paddle) without having to visually identify them.  Visual identification takes time, and isn't always possible in low light conditions.  This design input came from the famous gear guru, Military Morons.