It may well be whats called an "IF image" on your radio. add 21.4 to 105.8 and there may well be an ATC unit using that frequency (127.2 or very close to it if my maths is correct!).
To try and put it simply.. basically in the conversion process from radio wave to sound, an "intermediate frequency" of 10.7Mhz is commonly used in radio recievers and the image frequency is suppressed by several thousand times.. but occasionally there may be breakthrough if a transmitter on the image frequency is very close by and the signal from it strong enough.
There was a similar problem at times in the Bournemouth area and Radio One (98.2 +21.7 = Bournemouth Radar), and I would often hear aircraft breaking through on R1 frequency.
The Bournemouth radar frequency has moved now, but I dont know if that was a reason behind it.