Why an integer be subtracted from a bitstring mean PC-2 in arm pseudocode ?

Hi,

Where:

DDI0487G_b_armv8_arm.pdf

Appendix K14 Arm Pseudocode Definition

K14.4.4 Arithmetic operators

Addition and subtraction

Page:8584

Q:

There are two cases where the types of x and y can be different. A bitstring and an integer can be added together to allow the operation PC + 4. An integer can be subtracted from a bitstring to allow the operation PC - 2.

Why subtraction mean PC-2?

Thanks