2011_q3/2011_08_14_mismatched_bicycle_crank_arms

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A mystery - while doing some maintenance on my 2009 Trek Fuel EX-9 I had the cranks off for the first time, and I noticed that the crank arm lengths are mismatched. I haven't noticed any problems while riding, but it has clearly been this way since purchased. I wonder if this was the shop's doing or a manufacturing error?

Update 9/2011: a local bike shop was able to get Trek to replace the short crank arm with a 175mm part under warranty. I got stuck paying $15 for the labor to swap the part (which I would happily have done for free), but everything is back together now. Everyone I've spoken with agrees that this was likely an assembly error that occurred prior to shipping to the bike shop.

a close-fitting socket is used to align the left-side crank for comparison purposes. Left-side crank is clearly shorter than right side. view through the left side crank's pedal hole toward the longer right-side crank left side crank marked FC-M770/771-K 170 GE and right side crank marked FC-M770 175 GF