They're technical anomalies, transcending medium, legality and current technical standards to become something they were never meant to be.
However, 87.7 MHz (or 87.75 MHz to be exact), is/was the analog audio carrier frequency for VHF TV channel 6. Since the end of World War II until the DTV switchover in 2009, people who live in areas with a local TV station on Channel 6 could hear that TV station's audio signal on 87.7 on their FM radios, a fact not lost on the Channel 6 TV broadcasters (KHQ-TV in Spokane, WA promoted this for years.) And it was offered as a way to hear the audio portion of the Channel 6 TV station when you were in your car or away from a TV.
Bear in mind this wasn't a deliberate service the station offered. Just an anomaly of how the radio/TV spectrum was carved up. And unique only to analog VHF TV Channel 6 because the Channel 6 audio carrier frequency was coincidently in a tunable portion of the FM radio dial at 87.75 MHz.
However, in the early 2000s, several low power analog VHF TV stations began popping up on Channel 6. They weren't purposed as traditional TV stations, but as FM radio stations. This is why they are called "Frankenstations" An FM radio station using an audio frequency for TV.
The first Frankenstation was KZND-LP in Anchorage, Alaska. "87.7 The End" went on the air in 1999 and immediately outraged competing broadcasters who thought KZND was cheating and complained to the FCC. As it turned out, the station was using an overlooked loophole that allowed the audio portion of a TV channel to not be synchronized with a video image.
However, being an FM station on the TV band isn't as easy as one would think. First, you're technically a TV station. This means you must at least run some image on the video carrier. Which KZND was not transmitting, so the FCC forced them to start doing so. It wasn't enough the station had the ability to transmit a video image, but it had to actually do it to be within the law, as it was technically a TV station first. A simple graphic card to be broadcast over their video carrier was all the station needed to become legitimate.
Today, KZND now broadcasts on a real FM frequency (94.7.) 87.7 in Anchorage is now a jazz station called KNIK
WLFM-LP in Cleveland, Ohio actually used a Western Digital screensaver as their video carrier image!
Second, you have to be a lot more quieter than standard FM stations because you still must broadcast according to television technical standards. This meant a lot of the problems of a quiet uncompressed FM radio signal, such as "picket fencing", that "fwip-fwip-fwip" sound you hear on FM radio as you drive farther out of the station's primary service area is far more apparent well within the primary service area on an 87.7 Frankenstation. You can't broadcast in stereo either. While Zenith invented both FM Stereo and MTS Stereo TV transmission, the two systems are incompatible. All Frankenstations are mono.
And Nielsen Audio (formerly Arbitron), which measures radio ratings regards the Frankenstations as actual TV stations and doesn't count them amongst actual FM radio stations.
However all low power analog TV stations, which had been exempt from the 2009 American digital TV switchover must change over to digital themselves by September 1, 2015.
Which will mean the end of the Frankenstations because the digital signals can no longer be received over standard FM radios.
However since the analog to digital TV switchover there's been talk of expanding the FM radio band down to 76 MHz (similar to how the AM radio band was expanded from 540-1600 kHz to 540-1700 kHz in the late 1980s.) Which would incorporate the Japanese allocated FM radio band (which runs from 76-90 MHz) into the American FM radio band and allow American FM stations to broadcast on those frequencies. But that's only going to happen when the current FM spectrum gets so crunched, there is no alternative.
And we're already pretty much there in some parts of the country.