The water level was at the top a couple of days ago, so the lowered level is probably due to a combination of the flower stalks taking it up along with evaporation. This is the way an article by Southern Living magazine suggests that this is a good way to dry the flowers - just letting the water evaporate while feeling the petals to see if they are drying out enough - if not, you're supposed to add a little more water. https://southernlivingplants.com/inspiration-projects/step-by-step-drying-hydrangea-flowers/

Lots of serious perspective correction to this phone image.