Don: What do you mean by "average necessary social time required".
Marx: 'The labor-time socially necessary is that required to produce an article under the normal conditions of production, and with the average degree of skill and intensity prevalent at the time.'(1)
Say there are ten widget producers.
Nine of them make their widgets using hand-held hammers and an anvil, a process that produces one widget an hour. The tenth, however, has invented a steam-hammer that spits out twenty an hour.
Initially, the nine widget producers constitute the average time it takes to produce a widget.
The guy with the steam-hammer, nonetheless---or shall I say because of this---, is under-selling everybody and making a killing on the market.
Pretty soon, everybody else buys a steam-hammer (perhaps from an eleventh guy who has approximated the design) and within time EVERYBODY produces twenty widgets an hour.
Except, maybe, some peasant proprietor who still uses a hammer & anvil. This guy is on the fast track to bankruptcy because he can either sell his widgets at his 'average' cost (one per hour, or nine times the price everybody else sells them for) which no consumer would accept* OR he can sell at the REAL average (which would be his loss).
* See the middle of this post for more details.
1. Marx, Capital volume one, International 1967, p. 39.