David, Thanks for the quick response.
I do have a primary key defined. But my table itself has just 256 rows. I suspect for this small a size, postgres might have grouped several ids together which could cause this issue. The underlying concept which I gather from your post, is that postgres is keeping track of not only the data being accessed for modification, but also the path to access it. While my observations do support this, I never came across documentation which state this explicitly. Have I understood the problem right?
Fran.