Reading 'ARM Architecture' on Wikipedia and found the following statement:
Registers R0-R7 are the same across all CPU modes; they are never banked.
R13 and R14 are banked across all privileged CPU modes except system mode.
What does banking a register mean?
Answer
Register banking refers to providing multiple copies of a register at the same address.
Taken from section 1.4.6 of the arm docs
The term is referring to a solution for the problem that not all registers can be seen at once.
There is a different register bank for each processor mode. The banked registers give rapid context switching for dealing with processor exceptions and privileged operations.
If your looking for a more theoretical reasoning, I recommend this paper.
Edit: A much deeper answer than mine is given here