Bitcoin uses UNIX time for timestamp. Unix Time is really accurate. I want to know why there is a "70 minutes" offset is allowed in Bitcoin network adjusted time. Is there any distribution used to get to this "70 minutes".
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.