A photograph showing two thermometers with almost the same temp means that either one thermometer is off or that the air, water, and glass are all very close.
Water is very resistant (not conductive) to gaining or losing heat. This is why humidity plays such a massive role with weather (and why places like the desert can be so hot during the day and so cold at night while a rainforest stays relatively the same temperature).
It has been a while since I studied Physics, what exactly do the laws of thermal dynamics say that says that water will give up so much heat to glass better than air? Or that two thermometers, one in water and one on its container, will be as accurate?
After all this fluff and 'look what advanced topics I can bring up' are over, I still say don't waste your money on a tacky looking stick on thermometer from the 70s, there are better, more accurate options for about the same cost. My favorite cheapo thermometer is JW's Smart Temp Thermometer (white plastic with magnet mount). I think they should make it in black, but that is me. IMO
if you you are going to use a stick on put it on the side of the tank towards the back wall (where I put any thermometer anyways).
I am amazed I care enough to write all this. If someone wants to use an ugly stick on from the 70s go ahead.