The signal strength received by the TES can be affected by many things. Firstly, as with all electronics, there is some natural variation in the components and assembly of the TES. All TES, however, are tested and verified to comply with minimum operational standards before they are shipped. The received signal strength can further vary with the natural differences of the power of your Lionel transmitter, the quality of the wiring in your layout, metal or other blockage of the transmitted signal, etc. In short, any number of things can affect the signal received by your TES.