I suspect the information is cached with a time stamp of when it was last refreshed, so when you start during the day the information is recent enough that it'll be used from the cache, when you start in the morning it's too old and has to be rebuilt.
This is assuming your iMac isn't really turning itself off completely but is rather going to sleep while claiming to turn itself off, in accordance with the modern practice of lying to users "for their own good".
Reductionist and proud of it.
Being ignorant is not so much a shame, as being unwilling to learn. Benjamin Franklin
Chase after the truth like all hell and you'll free yourself, even though you never touch its coat tails. Clarence Darrow
A person who won't read has no advantage over one who can't read. Mark Twain