Megapixel count is used to measure still image resolution.

For video resolution, horizontal row count is used. For the AS100, the horizontal row count for highest resolution video is 1080. By standards, high definition video can be 720 or 1080 rows... at 16:9 aspect ratio, the AS100 can record a "video window" of 1920 x 1080... or 2,073,600 or about 2 megapixels... and can capture 18.9 megapixel stills.

On the X1000, 4k video also referred to as "ultra high definition" video has a "video window" of 3840 x 2160 and can capture 12.8 megapixel stills.

The video resolution did not go down, it (horizontal line count) increased. Only the still image resolution decreased.