It astonishes me that anyone would BE astonished that someone on the "left" could be a Christian.... ;-)

I've known a great many people on the left who had deep spiritual values, whether Christian or otherwise. Many of the early leftist leaders in Great Britain and North America were deeply religious, and many of them were members of the clergy. Leftist movements in Latin America have often been greatly assisted by members of the clergy. To be "leftist" is not necessarily to be atheist or non-religious, and Jesus himself appears to have espoused values that are much in harmony with a good deal of leftist philosophy.

On the other hand, I'm not at all astonished that people on the Right can consider themselves to be Christians...or Muslims...or Jews...or Buddhists...or atheists...or whatever. Why not?

It makes little difference, in fact. It's only in the USA that the Right has associated itself so strongly with a certain form of fundamentalist Christianity. Many people in the USA are perhaps unaware that their situation is unique in the World in that respect. It is not the case in the rest of the World that the Right of the political spectrum is so closely associated with what I would term the more radical or the stricter forms of Christian doctrine that have arisen in the past couple of hundred years (the "born again" movement, the evangelists, etc).