Your speed is increasing when going up hill because it is trying to stay at the speed you set the cruise control at.
So when going uphill, regardless of the cruise control or not, your car will need the extra speed to help it up.
Also going downhill your brake lights do not come on.
Since you have your car set to a specific speed on the cruise control, while going downhill your car catches momentum so it will downshift or not apply so much pressure because it is set to cruise at the certain speed you set it for, the internal computer knows not to go under or over that number it is engaged at.
Cruise control is no different than having your foot on the gas pedal, but the computer in your car just keeps the speed constant so you do not have to, because a lot of people have a problem with that and they think it saves gas. Sometimes the pedals are in funny spots I think and IMHO that can cause your foot to get tired and it is more difficult to keep the speed constant. My car now does not have cruise control although any other car I ever have had has had it, in the other vehicles I was glad I had it, the vehicle I have now I do not miss it at all, I can keep the speed constant with no issue. The pedal in my car now seems like it is positioned perfectly
Also in cruise control vehicles the only time the brake light comes on is when you physically apply your own foot to the brake pedal which disengages the cruise control.