The way that makes most sense to think about this to me is in terms of dynamic compression. At full throttle the turbo is shoving a large mass of air into the

The way that makes most sense to think about this to me is in terms of dynamic compression. At full throttle the turbo is shoving a large mass of air into the cylinder for every firing. The compression ratio has to be fixed lower to control detonation under these circumstances. At light throttle / cruise there is now hardly any mass of air in the cylinder, so although mecanically nothing has changed (same / fixed compression ratio) we are hardly compressing the air at all and we're giving away a load of efficiency for that reason. The ability to up the compression ratio at cruise / light throttle claims some of that basic efficiency back.