HELLO messages are exchanged between neighbour routers in a network at an interval of one-second. Occasionally, an interval will be skipped and there will be two seconds between HELLO messages sent from a router.
This is caused by measuring the time-since-last-hello to determine if a new HELLO should be sent. If, for some reason, the invocation of the "tick" method in the router engine happens just slightly less than one second after the last tick, the HELLO won't be sent until the next tick.
This can be seen by enabling trace+ for ROUTER_HELLO logs.