Stroked line climbs the page while I'm adjusting its length

Here's my experience:

• I draw a line with the line tool, stroke it, and position it where I want it.  No prob.
• If I should need to adjust its length, I choose the appropriate end on the proxy to lock in the direction toward which it will adjust, and then adjust the length numerically in the length field, point by point, so I can get it exactly right.
• The stroke changes length just fine throughout the process, but it also simultaneously creeps upward and a little to the side, toward the anchored end. Basically, the line doesn't stay where I put it.
• I thought it might have something to do with the object scaling settings in the General tab of Preferences, but I'm not sure. I've attached a screen shot of my current and most common settings of this preference.

I've noticed this behavior many, many times and usually just drag the line back to where it belongs and keep going with my work. But I thought maybe it was time to ask about this to find out if anyone else has had this experience and if you know why it happens or how to make it stop (or what I might be doing wrong!). -- Thanks much for any feedback. 🙂