Each instance of a CS skin is assigned a random amount of wear between 0 and 1, so two copies of the same skin can be worth more or less money depending on their condition. To be clear the value is fixed, actually using a skin won't make it dirtier. Factory New is the highest tier with a wear value between 0 and 0.07.
The game itself only distinguishes between those ranges of values, but it's possible to query the exact number via an API so I think traders will even price that in (e.g. Factory New 0.02 is worth more than Factory New 0.06).
