I'm going to presume there was a competition reason why Radio 1 didn't get lumped in at the bottom end of the FM spectrum with its peers, as you have that entire 88-95 section for the BBC stations, a gap for commercial networks (or some local BBC stations in some areas depending on where Radio 4 ended up), Radio 1, Classic FM and the rest of the spectrum for everything else.
Because the FM band was smaller than it is now. The sub bands used by Radios 2-4 date back to the 50s when it ended at 94Mhz. Then the new local stations in the 60s and 70s went above them up to 97ish Mhz. The next bit allocated was 100-106 in the late 80s early 90s and the next wave of stations appeared there as well as being extra/new frequencies for the older BBC locals and ILR stations.
When Radio 1 went onto FM in the 90s it went into the 97-99 gap which until the late 80s had police radio on it. It couldn't go above Radio 4 because there were stations there already and there was no room below Radio 2.
Classic got 100-102 but there are some locals in their band too, some of these pre dated Classic such as Kiss 100 in London. There are fewer oddities in amongst the BBC's
Incidently the last block of the band to be released for broadcasting was 106-108. When it was being decided what to do with this there were calls for it to be a new commercial national network, in particular what was then Virgin Radio were eyeing it up. It didn't happen though