Reason that the brake fluid levels change is ... when the pads wear the pistons stroke out and the fluid level drops .. diligent owner/mechanic then tops back up to max level .. then pads need changed.. basic mechanic then pushes pistons back and fluid level goes back up and then may be close to overflowing. Good mechanic takes a cupful of brake fluid out at calipers as the bit that is most contaminated with water is at hoses .. and it keeps the brake nipples free.
So fluid levels change with pad wear .. temperature not too relevant.
And change it as a min every 2 years... and brake fluid in the can is no use 2 years later.. it absorbs water from the atmosphere .. so just push 80% through the brake system when you flush/bleed and keep the last little bit for a top up if you really need it... which if you do .. it normally means problems .. so if you are using brake fluid.. look for the
problem NOW.