track tempo slightly changes when i render it

[leandro]

well.. this is very simple... i open a full lenght track (mp3) on live, i chop it in pieces and move them around (basically, im editing tracks in easiest way... nothing from out of this word)
(i put warp markers every 8 bars)
i render this.. and then, i drag the rendered wav (or aiff, i try this both on mac and pc) to this same project on a new channel, right below the channel with the edited mp3. and guess what ? i zoom in and the beats doesnt match.
unfortunatly, i noticed this playing... i felt that beatmatch wasnt ok between some tracks... so i checked on the edited projects and this camed in! some tracks are rendered in a sligltly bad tempo. how is this happening ??
any of you with the same problem ?

btw, im not using any plugins, effects... absolutly nothing... just an mp3 on live, thats all. (im in live 5.03)