If you see below in man 2 brk you will see in NOTES:

The return value described above for brk() is the behavior provided by the glibc wrapper function for the Linux brk() system call. ...
However, the actual Linux system call returns the new program break on success. On failure, the system call returns the current break.
The glibc wrapper function does some work (i.e., checks whether the new break is less than addr) to provide the 0 and -1 return values described above.