A while back I saw this subject mentioned (I couldn't find the thread). Allegro adjusts the coordinates when rendering text so that it will always be drawn aligned to the nearest pixel. I'm not sure why it was implemented this way, but I'm sure the developers had a good reason.
I am having a problem when I use transformations while rendering text and other graphics, or more generally, when drawing moving objects that contain text. The text movement and scaling appears jittery compared to the rest of the graphics.
When this issue was brought up before, it was mentioned that a way to disable the coordinate adjustments might be added. I haven't heard anything since.
While looking at the code I noticed that al_draw_justified_* does not do this integer pixel alignment. This means there is an inconsistency in the text rendering functions.
I don't see why the integer pixel alignment should be done at all, myself. The user should be doing that in their own code if they desire it. If we did away with the integer pixel alignment altogether, that would make the library's drawing functions consistent and easier to understand.
At the very least, there should be a way to disable the integer pixel alignment via a flag passed to the text rendering functions. As it is, I'm stuck having to come up with a convoluted solution to get my text and objects to line up and move smoothly together.