Trackpad click-drag to open and select action from menu doesn't always operate on originally selected area

This is probably operator error, but the behaviour seems incorrect to me. I'm trying to make features of individual bases in individual sequences in a DNA alignment.

1. I have nothing selected (and nothing up my sleeves...).
2. I click on the base I wish to make a feature of. This is highlighted as expected (fig1.png)
3. I right-click the highlighted base and select selection -> create sequence feature. The highlighted selection expands to a seemingly random size (fig2.png)
4. I click 'ok' to create the feature.
5. I end up with a feature way bigger than I intended (fig3.png).

On some occasions I end up with the 1 base feature I'm after, but more often than not it spans multiple bases on multiple sequences. I then end up going through trimming the features back to the size I intended.