I had seen a post (Continous transforms accumulating delay, can be tweak for speed up? - #7 by Germain_Pavot) in which date_histogram was suggested as potentially impacting use of date in the transform searches.
You're right, I do see in the searches a set of other terms taken in account that seems to vary with updated documents.
Thanks for the explanation; that makes sense. I just wanted to be sure I hadn't done something wrong.