Realistic explanation?
Same way you can wear a shirt and a coat!
The armour categories are pretty generically defined down to single items, as I see no real purpose to have separate armours with ever so slightly different stats. The bonuses are all low and require using multiples to stack up. In a databasing sense it's just cleaner and more logical to keep the gear to one set each (armour: def, helmets: mdf, boots: agi etc) and just rely on artistic licensing for the immersion quality:
it's not actually two of the same armour you're wearing: it's one shirt and one bulletproof vest! :D