Performance of UNION vs IN

I have a very complex view zinfoexp and running the view as:
SELECT * FROM zinfoexp  WHERE idmembre in (1,84)
take 2700 ms

So, I try another syntax:
SELECT * FROM zinfoexp  WHERE idmembre = 1
union
SELECT * FROM zinfoexp  WHERE idmembre = 84

and for me, two calls to my view takes a lot of time (may be x2) and it
takes 134 ms !

How is it possible that the optimizer cannot optimize the IN as UNION ?
I have a database postgresql 9.3  freshly vacuumed

--
Jean-Max Reymond
CKR Solutions Open Source http://www.ckr-solutions.com