the reason is most likely because the longer throw on the crank (on the 2.1, 2.35 and 2.5 engines) pushes the oil control ring past the gudgeon pin inspection hole.. if the engine happens to stop at that point when you switch it off which is highly likely, it will allow a small amount of oil to drain past the compression rings into the cylinder.. you start the engine and you get a puff of smoke.. if it lasts for more than an initial cloud then I would consider it more than just that and would need investigating why.
The fact it is not using oil or blowing out the breathers would say it is nothing to be concerned about.
Simon