Schmoekoeksklok (i think i got that right) is right. You hower need to keep in mind that Diplomatic reputation is determined in whole numbers rounded down. Example: 2.95 dip rep. will give +2 to progress. Your diploannex progress is: base +1, same culture +1, same religion +1, +X dip rep.
Dip rep is modified by: Dip rep ideas (diplomacy and influence groups), legimacy, overextension, advisors, NIs AND if you have recently diploannexed someone (gives -3 for 10 years). Im sry if i forgot something that modifies it.
You get "Unknown" if your diploannex progress is 0 or <0 as there wont be any progress then.
EDIT: as Steelensturm say if you start the progress you can hover the progress bar and see a breakdown of your +'s and -'s that modifier the progress.