ST60 Tridata interfaced C80 multifunctiondisplay

Hello, I have A80171 version of the P319 fishfinder transducer interfaced to the DSM300 -C80 multifunction display and Tridata ST60. The depth is the same in C80 and tridata ST60. The temperature in C80 is 29 Celsius and in Tridadata ST 60 is 23 Celsius. Why this difference in temperature? The connection between multifunction display C80 and the ST60 tridata is seatalk.
Thanks,
Sergio