Hey Bart,
By excluding the 1st trade would mean that the gross loss/gross profit of the 1st trade would not be included in the computation of the profit factor over a particular strategy AFTER the 6th trade has taken place.
Thus, in this case, even though there are a total of 6 executed trades over a particular strategy, the profit factor will be evaluated based on the most recent performance, albeit, the past 5 executed trades (with the 1st executed trade being excluded from the computation since it does not reflect the most recent performance).
Cheers,