Hello
Firstly, you should consider that, in order to analyze the MRP results, it is always better to use transaction MD05 than MD04. See the blog Why should I use transaction MD05 to analyze the MRP results? for details.
You should also take a look on the following wiki where the rescheduling is described in more detail:
http://wiki.scn.sap.com/wiki/display/ERPMan/What+is+rescheduling+and+how+does+it+work
Please read the note referenced on the wiki for more information about the rescheduling check.
BR
Caetano