Better FM reception after adding XM radio

Here's a weird thing. We recently moved to the CO mountains and immediately experienced the lack of radio reception on our Prius. We couldn't receive any FM channels in the Prius while my GMC pickup sitting next to it in the drive could get 4 or 5 channels.

I finally got fed up and bought an XM receiver. During installation I put the puck antenna on the roof just above the passenger side corner of the hatchback lid. The instructions warned of getting it too close to the radio antenna and said it should be at least 3" away. After I finished the installation and receiver activation I was sitting in the car, in the drive, setting all the preferences. Out of curiosity I switched the car radio back to FM and immediately noticed I was receiving a station I normally only pick up when I get about 10 miles closer to town. I started searching and found that I can now pick up the same 4 or 5 channels I can in my GMC. The only thing that's changed is the addition of the XM radio. Is it possible that the additional XM antenna helps the FM radio? What do you experts think?

I'm stumped but happy about the mysteriously better reception.

 

I could see that if I had fiddled with the OEM radio. This is a totally separate receiver. Signal reaches the OEM via the AUX input. Not attached to the OEM radio at all. It's very strange but I'm happy about it.

 

I never touched the OEM radio. I installed XM's OnyX receiver. Totally separate from the OEM radio. I have the option of sending XM's signal to the OEM radio via either the OnyX's built in FM transmitter or thru a AUX cable. I chose the AUX cable as that gives better sound. I simply plugged one end of the provided cable into the back of the OnyX receiver and the other end into the AUX in receptacle which is located inside the center console.

Has to be something about the additional antenna but I can't figure it out. Not complaining just curious.