Hello,

It could be to do with the summary itself and which router originated it. When you create a summary address (regardless of where it came from) the router you are implementing the summary on becomes the new originator of the route suppressing the more specific (Type 5 includes) routes. So since it is originating the summary route it will have it as an O route on the device. With summary address keep in mind there is a loss of path information as the newly created summarized route suppresses all of that. You wouldn't see the type 5 route because its being suppressed in place of the summary as a whole. 

May have to lab this up in a bit.

Hope that helps.

-David