I would agree with RDiaz that the cheaply made suit gives itself away in motion. The chest and lapels are glued, and the prefabricated shoulder pads are rigid. The jacket does not move smoothly with the body, but stands stiffly away from it.
I had the same reaction to that grey fused suit:
Not all expensively made suits are well-constructed, but those that are have soft, hand-sewn collars and lapels. The shoulders are more natural, and the chests more flexible. The garment is held together by stitching, not by fusing. As Luciano Barbera says, you should be able to play golf in your suit. If you look at Slewfoot's pictures of his Steed sportscoat by Edwin DeBoise, there are a few where Slewfoot is running in a backyard, kicking a ball. His jacket collar and shoulders move fluidly with him.
So why do cheaply made suits sometimes look good online or in a catalog? It's because the shots are standing still, the jackets are pinned, and the pictures are photoshopped (as a friend who edits pictures for a catalog tells me):
I say cheaply made, because some suits have low-end construction, but high-end prices. But, what about fit? Can't a well made suit be poorly fitted? Yes, but good construction makes it possible for a suit to fit well in motion; it does not guarantee a good fit. It's like how fuel makes a fire possible, but you also need oxygen and heat for the fire to start.