Page "Labor theory of value" Paragraph 21
The value of a commodity increases in proportion to the duration and intensity of labor performed on average for its production.
Part of what the LTV means by " socially necessary " is that the value only increases in proportion to this labor as it is performed with average skill and average productivity.
So though workers may labor with greater skill or more productivity than others, these more skillful and more productive workers will thus produce more value through the production of greater quantities of the finished commodity: each unit still bearing the same value as all the others of the same class of commodity.
By working sloppily, the unskilled workers may drag down the average skill of labor, thus increasing the average labor time necessary for the production of each unit commodity.
But these unskillful workers cannot hope to sell the result of their labor process at a higher price ( as opposed to value ) simply because they have spent more time than other workers producing the same kind of commodities.