Apparently 4 of the pixels in level 15 of the Structure pack decided to take some inspiration from level 12 and went on holiday and never came back. They have now been fired and replaced.

IMPORTANT: If you have completed level 15 of the Structure pack your total pixel count will be 4 less than it should and this will prevent you from getting the paint all pixels achievement later. You have 2 options: 1 - You can reset the level and do it again. 2 - Please let me know and I will walk you through a work around.