Pooldv, you are incorrect they tend to all be 10K sensors, but some have different curves so while they all read 10Kohms at 77 degrees F, the farther the temperature is away from that the more they may read wrong. This can be a big problem when it comes to freeze protection as it can shift the readout temperature by about 15-20 degrees one way or the other depending on which curve the individual product expects.
Ike
p.s. this is usually a minor issue if dealing with swim temperature measurements as the difference is only a degree or two at typical 85-90 degree swim temperatures.