The unary -a and -e operators in test are identical in bash.
If you want more backstory, check the POSIX test(1p) man page:
but there was no easy way to determine that an existing file was a reg‐
ular file. An early proposal used the KornShell -a primary (with the
same meaning), but this was changed to -e because there were concerns
about the high probability of humans confusing the -a primary with the
-a binary operator.
You seem used to give useless information like unix backstories. Only
unix nutcases delve into that.
The POSIX test command only supports the unary -e operator, and does not
list a unary -a operator. (And the binary -a is considered obsolescent.)
Gnu Bash could moves on and stop supporting and maintaining -a.
Here is what bash reference manual says -
6.4 Bash Conditional Expressions
-a file
True if file exists.
-e file
True if file exists.
How is anybody able to figure out anything from that !