Because the temp property is stored in saves as an integer on the Kelvin scale (unless it's less than 128K off from room temperature, in which case it's stored as a signed 8-bit offset from that).
If you're having a separate problem related to this, maybe we can solve that problem instead. There's no way to make temperature not decrease by .15, it was deemed "good enough" to save temperatures with just 1C degree of precision.