I used to develop my own slide film.
On the other hand: ;)
Using your argument, the vessel itself, being cooler than the hot water, will have cooled the water slightly at lower levels in my measurements.
The hot water stream as it enters the cup will indeed be somewhat effected by your "ambient" air temperature on the way down to the surface, but, without an additional source of heat, and until the vessel reaches an equilibrium temperature with the liquid as your sink did, the temperature of the water stream at the entry point will still be at its highest temperature ... and that point of entry is not within the scientific definition of "surface temperature" of a liquid.
IIRC my physics classes, "surface temperatures" are not necessarily lower than at lower depths, quite common in boiling water with the nucleate film effect that vapor causes.
;)