Because the zram bucket is compressed and thus requires typically half the amount of real RAM.So if you have 1GB of RAM and set up a 512MB zram for swap, those 512MB when completely used up will typically only consume 256MB of RAM. So you will end up with 768MB of actual RAM, + 512MB of swap = 1.25GB of RAM instead of 1GB.Tweak ratios to suit your workload, but that's the basic gist of it.