Sounds like it's actually controlled by a programmable t-stat. (Does
your manual t-stat actually control the temp setting at other times?)

Or it's haunted.

Sorry, but your limited understanding of the system, implied in your
question, means that you need to call in someone who can answer your
questions. Provide a full description and list of questions up front,
or arrange to be there to ask personally.

And ... you would benefit significantly from a programmable t-stat,
using setbacks (night and mid-day) as deep as possible. Small price
differential from fixed-setting oldies.

J