Probably expansion. Back in the day (pre-1970) cars did not have expansion tanks
and you learned to leave the af down ~ 1 1/2 inches.
And the reason cars have overflow tanks
is NOT to recover the coolants, but to exclude oxygen and reduce corrosion
On the other hand, frequent af changes on marine
engines are not a real bad thing. While long life antifreeze
can easily go 5 years, most marine
engine manufacturers require more frequent changes because of the risk of seawater contamination... though I would simply test for chloride ion (not chlorine).