The picture is starting to come into focus a bit more… as Eric continues to whine and wonder why all the attention isn’t on him… he’s here, after all, and hasn’t run away…
The look on Ellie’s face though… She not happy… Almost as if to say, why did I marry into this cause??
The Daily Trail 
I’m going to get kind of art-geeky here: Interesting inking in these panels. First, Mark’s face has some kind of woodcut hatching, but randomly across his face, not following any kind of lighting source. And it is flat, not countered around the face, and stops randomly, rather than naturally, such as around Mark’s ear. And a rather un-Mark face, to boot.
Eric’s face, at least, is more consistently shaded with regard to a light source and is a “shade” more contoured to the face.
Meanwhile, Frank’s mug has no shading at all on his face.
And even more geeky, the shading/inking style of each of their shirts is different. Does this represent different material, artistic variety, or different inkers? This is uncommon.
Perhaps Allen is playing jokes on his readers. I understand he pays attention to reader comments on the various comics sites. And some of his strips over the past few years have poked back at some of his critics. Maybe that accounts for the hodgepodge of styles we’ve been seeing lately. If this theory is true, I’ll tip my hat to him.