Update on tables when the row doesn't change

Hi all,

maybe it's a very silly question, but why does Postgres perform an
update on the table even if no data changes?

I recognized this recently doing a rewrite of my rules because they
took to long. I had many conditional rules I collapsed to one
unconditional rule, so that the views get constructed only once. If I
split these updates to the underlying tables, I get a lot of updates
which don't perform any "real" updates.

Can I circumvent this behaviour of Postgres only by defining lot of
rules / triggers on these underlying table are there some trickier ways?

Any help appreciated,

Sebastian