Going by their shipments for September then they can only really mean shipped, which isn't surprising as it's not uncommon for manufacturers to use "installed base" to refer to shipments. Anyway, 2.5m expected to be shipped during first two months of Q4, I wonder how many they can ship in December.
Interesting Europe numbers (+62% month over month), although 360 hasn't been outselling PS3 in US two to one (last month it outsold PS3 by three to two), maybe they meant LTD, where 360 has sold around twice as much as PS3.