Because your off hand damage is already reduced to 50%, adding 50% at level 5 does not make 100%, it makes 75%. (0.5 base + (0.5 modifier x 0.5 base) = 0.75 total)
Tips & Tactics
Dual Wield Specialization is one of the best talents in PvE. The DPS increase gained from taking this talent is very high. This talent is a must have for a raiding Rogue.