What Use Is An F-call?

What use is an F-call? I've recently purchased four verticals, one for each of the HF bands I'm allowed to use. I installed them and started playing, only to be confronted with some interesting results. The 80m and 40m verticals have a very high Q, that is to say, they resonate on a particular frequency and you can make contacts at those frequencies with about 22 kHz variation. This is as expected. My antenna analyser picture looks just like the one on the box. This is great. In theory all I should need to do is trim them a little bit and have them resonant at the frequency I want to operate on. I did hold off on the trimming, since cutting antennas longer is pretty hard to do, and because I got some weird results for the other two verticals. On 10m and 15m the antennas are resonant outside the band. On 15m it's below the band and on 10m it's above the band. I could just cut the 15m antenna shorter, but there is something strange going on here. I tried using different mounts, even a magnetic - not recomm