import stills duration vs media length

I set my stills/freeze frame duration to 1 second. I imported a bunch of jpegs. I want to batch export them as short prores movies (1 second each). I set up a batch export with my prores settings and hit go. Every clip comes out with a duration of 2 minutes.

Hm... I look in the batch window, and sure enough, while the duration of each clip says 1 second, there's another column, called Length, which shows 00:02:01:01.

Where does FCP get this number, and how can I override it?