Why is This Comfortable?

I will never understand cats.  But they are wonderful to watch because they do so many interesting and cute things.  This is Grace, the abandoned kitten that we saved.  She is really growing as fast as a weed - she is now really long and thin and athletic.  Her ability to jump long distances is astounding.  I saw her like this and couldn't imagine why she thought that this was a comfortable way to rest.  Her tail end is on the window sill, and there is a gap large enough for the radiator, and then her head is on the easy chair.  I cannot imagine that it is comfortable, but she stayed like this for quite a while.  Or at least long enough for me to capture this in a photograph.