In the event that a Node does not meet minimum availability targets, penalties will be applied against their POS. You can think of this as an uptime guarantee. Uptime is more important for Stargates than for Gateways and for Gateways than for Hosts, and the availability requirements are set accordingly. Stargates must meet a 99.9999% availability, whereas for Hosts availability is set at 20%. Availability can be low for Hosts because of how the system is architected: it is designed to allow for a high level of disconnect and reconnect at this level.