"Trim" is something I'd never heard of... maybe that's all this is? I didn't know there was a way to compensate for cross-chaining! (I just try not to) It's a first road bike for me, so maybe I just need to press harder on the levers when I want to shift.

Thanks for the advice, I had a look at the tech docs. This seems to be beyond my DIY skills, and I don't have 'the special tool' (seriously, that's what it seems to be called) so I'll finish my tune-up (the limit of my DIY skills), and take it for a ride to think about this 'trim' thing some more. If it's still bugging me I'll go back to the LBS to ask about it, and also about something that flew out of the rear shifter on a shift during one of my first rides. I don't see any obvious missing bits, and it seems to work fine, but it's making me nervous. I think a return visit is in order.