Why does RPM drop sometimes in cruise control?

I have rostra cruise control on my Ford Fiesta. Usually when cruising at 75 mph, my engine will run at about 3000 RPM. When going down hill with the cruise set, sometimes the RPM will drop to a little below 1000. This is equivalent as if the engine was running while the vehicle wasn't moving. After the downhill ends, the engine will rev up and usually will end up changing gears to stop the rev. However, one time it didn't stop and my car went to the rev limiter in the redline at about 7300 RPM. No matter how hard I try, my car will not let me into the redline when I'm controlling it. However, with the cruise engaged, this did happen. This doesn't happen very often, it has only happen like 4 times since using the cruise probably 100 times, and only went to the redline once. If I tap the gas pedal while it either drops to below 1000 rpm or while it is revving up, it will bring back RPM back to the normal 3K. However, if I don't, it will rev up usually to about 5-6k before changing gears, in exception to the one time that it over-revved. I'm wondering what is causing this or if it is normal for cruise control. I'm also wondering if the RPMs dropping so low is normal, because like I said, usually it will stay around 3k but every now and then it will drop below 1000. The car is a 6 speed automatic dual clutch transmission, so the gears are controlled by the car's computer. The rostra cruise control is an aftermarket cruise control unit.

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