simple.....u depart having 1,2 or 3 reversers inop. on approach briefing, u mentioned that u wanna use idle reverse or no reverse, or max reverse on remaining reverser coz the runway is long and dry. so check on QRH 4.03 actual landing distance config full, dry rwy, without reversers correction.
this statement :
A/ USE the LDG DIST factors “without REV” when:
1. All reversers are inoperative or
2. Maximum reverse thrust on all available reverser(s) is not selected or
3. The aircraft has been dispatched with one or more reverser(s) inoperative
B/ USE the LDG DIST factors “with REV” when:
1. At least one reverser is operative and maximum reverse thrust is selected at landing
this only applied, if u are landing with a failure/s that were mentioned in the failure tables, QRH 2.41, 2.42 and so on. there's a note below the table that says, if u dispatch with 1 or more reversers inop, use the actual landing distance (QRH 4.03) without reversers correction, regardless on actual reverser application on landing.
hopefully this will shed some light in your understanding...