Typically when this happens, the top outlet is controlled by a switch. This is common in bedrooms and living rooms where there is no ceiling light fixture.
The receptacles come with a tab on the side that connects the upper and lower outlet which has likely been removed. This allows the connection of two different hot wires, one to the bottom outlet and one to the top that is controlled by a switch. Please refer to the connected home offer for more details.

Do X Series Wiring Devices support copper-clad aluminum (CCA)?

Released for:Schneider Electric Canada