Pipelines
The ARM code implements what is known as a pipeline to save time. The CPU is always attempting to fetch an instruction, decode an instruction, and execute an instruction at the same time. The pipeline for ARMv5TEJ is 12 bytes (4 bytes for FETCH, DECODE, and EXEC). This means that the value of PC is 12 bytes off from where the instruction thought it was when it was fetched. When operating on PC, PC is the fetched PC. It’s more likely you want PC-8.
Different architecture versions have different sized pipelines. This means that if there is code that assumes that PC is 12 bytes off in one program, it won’t work when running on an ARM architecture with a different pipeline size. For example, many will assume ARMv5 code should work on ARMv7, but this isn’t true in all conditions because of assumptions and constraints like the pipeline size.