I can tell you what the catch is :S
The Levis on Amazon are authentic however... there is a vast quality difference depending on where they are made - I can't remember the exact countries but its something like the ones made in China, Bangladesh or Colombia are lower quality than those made in Portugal, Turkey, United States, Japan or Italy if you buy from say Debenams or HoF, etc. they will almost always be from one of the good factories whereas on Amazon its a complete lottery.
This, plus Feek's post.
The same thing happens with a lot of the stuff for sale in TKMaxx and the like. Branded, but inferior products aimed at a lower segment.