Low memory vs total memory in "free" command

I am trying to understand the output of the free command on an AWS Linux server. For example, free -h gives: total used free shared buff/cache available Mem: 15G 2.2G 4.0G 16M 9.0G 12G Swap: 0B 0B 0B Whereas free -hl gives: total used free shared buff/cache available Mem: 15G 2.2G 4.0G 16M 9.0G 12G Low: 15G 11G 4.0G High: 0B 0B 0B Swap: 0B 0B 0B How can the "Low" used memory (11G) exceed the total used memory (2.2G)? From everything I'm reading, -l is supposed to split the totals into totals for "low" and "high" memory, which isn't even a distinction that's supposed to exist on 64-bit systems. What does the 11G actually mean then?