'const' has the meaning dictated by the C language standard.

The other (should have been __code) is the one Keil is free to define the meaning of.

'const' is only a definition of your right to modify a variable. It does not say anything about storage. It is 100% up to the compiler vendor to decide if a 'const' keyword should make any storage difference, as long as the semantics of the code stands. That is why it is forbidden to make 'const' equivalent with the code segment - 'const' must be allowed to be used in situations where 'code' does not make sense. 'code' can't be used with auto variables. 'const' can.

'code' is only a definition of storage. That it will imply read-only access is just a local peculiarity of the 8051 anatomy. Storing data in the code segment of x86 running in real mode would not imply read-only access. Moving to x86 protected mode, the text segment would most probably be write-protected, and result in a segmentation fault.

In the end, storage and access attributes are completely separate, but for specific targets, a specific storage may imply a specific access, while the reverse is not true. It is a compiler vendor decision if a specific subset all 'const' declarations should be automatically upgraded to also imply alternative storage - it is neither implied nor required or forbidden by the standard.