The State of Florida in 1998, in order to fight disparity in sentencing, created a "scoring system"[0] where every crime in Florida is given a number that corresponds to the number of months in prison that crime could carry.

The scoring system is byzantine as all hell and I challenge anyone to figure it out.[1]

"Scoring out" is simply a term used by Florida convicts to explain how much time they got among themselves I guess. Maybe it was inappropriate to use it here.

Gain-time is a sentence reduction scheme where people get time off their total sentence for staying out of trouble.

[0] http://macklawpa.com/criminal-defense/felony-score-sheet/

[1] http://www.dc.state.fl.us/pub/sen_cpcm/cpc_manual.pdf

[edits]

I am guessing a bit but I am understanding 'gain time' to be accrued time.

I think 'score out to' was 'sentenced to'.

If I read it right they are saying (mostly) the time spent in jail does not accrue days off a sentence like the time spent in the prison would.