Check for bubbles in the bulb from "cooking" the liquid, in the bulb of the thermometer. There is also a very sophisticated explanation for it. It is called Contact angle hysteresis. That is where the liquid/glass/vapor interface experiences total resistance to movement. Interesting reading about that [https://dspace.mit.edu/bitstream/handle/1721.1/86387/Eral_Contact%20angle.pdf?sequence=2&isAllowed=y|on here]