I started a regular tuneup on my gf's 7-speed roadbike today and out of habit checked the chain stretch. This bike has (probably less than) 400 miles since the last chain replacement. To my surprise it was obviously .75 stretched! I talked to my LBS mechanics and they told me there's almost no way this could have happened in less than 2000-3000 miles and were confused. Any ideas what's going on?

as a side note:
-the cogs and chainrings have been maintained
-rides in all weather
-live in new orleans, the roads are flat so we often ride in top gear only
-chain is lubed occasionally, but probably could be lubed more often
-chain is not rusty or dry