Network Time of Bitcoin
The leeway exists mostly because timekeeping is not very synchronized, overall. Computers can easily be off by several minutes, and around daylight time changes it's not uncommon to see computers off by an hour or so.

Having stuff like NTP makes things more in-sync, but one cannot expect all participants in the network to be using services like NTP.

Counting up from the Unix epoch is very accurate, yes. The ability of various computers to maintain that accuracy is not.