Some years ago I bought a duvet which said "[This duvet] is designed to keep you cool in summer and cosy throughout winter". I didn't think much about it at the time because it seemed like typical advertising bluster, but then after using the duvet it's become apparent that it actually works. I need to wrap the duvet tighter during winter (because any exposed body part definitely feels cold), but aside from that, it is OK to use the duvet during both summer and winter.
Thermodynamically, how does this work? Naively I'd have imagined that something that keeps heat in (important during winter) would also cause the user to overheat during summer - which is why winter jackets are not worn during summer.
If it matters, the box says the composition is "outer: cotton & inner fill: polyester".
Related: How do blankets keep you warm? which however does not deal with the question of why the blanket works during both summer & winter.