Like Paul said, interpolated and interlaced are two different terms.

Interpolated means (very roughly) that with pixel a and pixel b in any given image (still or video) the computer makes up pixel c between them to make the image bigger. or averages them together to make the image smaller.

Interlacing is where video of a given frame rate is split up into lines that are sorted as odd or even, then displayed as a group so that only half the image need be displayed at any one time. It is as I said a throwback from Cathode Ray Tube televisions, but it is still used today as it can halve the amount of data required in a digital signal.

Yes I know the same total amount of data is there, but if the processor takes extra time to process the extra data splitting it up into manageable chunks can really help.

In this particular case, the camera is using the fact that by interlacing the image (and only needing half the resolution) its processors can handle 60 frames per second as opposed to 30. Incidentally this has a flow on effect to the computer which is struggling to display 30 full resolution frames but can easily display 60 half resolution frames.

in photographic terms we are swapping 30 8mp fphotos for 60 4mp photos. Much the same as the digital slr's with the fastest burst rates have slightly less resolution to allow the cameras CPU to handle all the data.

Again it links in with hard drive and computer bus technologies. with video sustaining the data rate is more important than short bursts of peak capacity followed by a lag.