To simplify the interpolation math, the look up table includes the cosine of every fourth degree (0°, 4°, 8°, and so on) rather than, say, every fifth degree (0°, 5°, 10°, and so on).
This makes the difference between the known upper and lower angles four, and allows the division of the difference between the cosines of the known lower and upper angles to be accomplished with two right shift instructions.
Similarly, because all the known angles are multiples of four, the difference between the given angle and the known lower angle is found by logically ANDing the given angle with three.
These simplifications allow the interpolation to be taken with only three mathematical operations:
1. the difference between the cosine of the known angle and delta
2. the difference in the numerator of the delta fraction
3. the product of the two delta terms