Select by Location / Within results are odd

[Craig_Eissler_Iceman]

Below is just a simple Select by Location requesting Buildings "Within" the Floodplain.

Why would I get the odd results below, with some buildings not getting selected?

Thanks

[MrRamonG]

Could the buildings that weren't selected be parts of a multipart polygon feature? 

[Craig_Eissler_Iceman]

When I select or identify one of those 'unselected' buildings, nothing else is selected or listed -- if that's the way to check..?

[Craig_Eissler_Iceman]

For anyone who cares, I figured out that the reason for this result is because there is more than one (two, in this case) polygons that represent the 'Select Features' input. Therefore, any 'Input Feature' that may intersect the polygon line dividing these will NOT be selected if your spatial operator is set to 'WITHIN'. ...So, even though it's Within the 'Layer' set as the 'Select Features', it doesn't matter.