Not all events have an _t property. Some events are not from the boards themselves but from actions taken in the api like naming troops.
Where there is an _t property it should always === the time property.
functionally _t is the server timestamp as soon as the "board server" process aka "carlo" receives the sensor report from a scout. It was left in for debug, and may not exist in the future. It was intentionally undocumented, but I'll add an entry to the documentation that explains it though rather than leaving it a mystery =)
sorry for taking so long to respond i totally missed this!