100% of the 160m band and about 90% on 80m, the tuner takes care of the rest
I think CK pretty much summed it up. ground losses.
A vertical at resonance should have around 33 to 35 ohm impedance, 1/4 wl vertical that is, the ground radials are not bent at a 45 degree angle to get the 50 ohm impedance match.
So considering you have a 1/4 wl vert and you have that much band width you have tremendous amount of ground loss.
I know you make contacts and it works etc etc.
I put up an 80 meter vert and had one radial on it. First contact was into croatia with a 100 watts. I really thought I was doing something.
After i did more research and laid down around 24 radials my impedance went down, VSWR went up and band width decreased.
But all that really matters is that you are enjoying your antenna and making contacts.
It can be more efficient.
Add more ground radials, or enjoy the band width and get some gain from turning on an amplifier.