Why Linux read() avoids using full 2 GiB in one call

Reading the manual page (man 2 read) on my Debian system, I see the following note: >NOTES >[...] >On Linux, read() (and similar system calls) will transfer at most > 0x7ffff000 (2,147,479,552) bytes, returning the number of bytes actually > transferred. (This is true on both 32-bit and 64-bit systems.) I am guessing something is being done with the remaining 0xfff (4095) bytes so that this number is chosen for a specific implementation detail. So my question is: Why is it 0x7ffff000 and not simply (naively) 0x7fffffff?