Sgt_Kickaxe - 9:11 am on Jun 23, 2012 (gmt 0)
For every 1000 visitors I receive from search to any given page they come in on 900+ different keyword variations. For that reason alone I can tell you it's not possible to claim you hold the same keyword rankings, there are too many to check.
If your page was downgraded for whatever reason, by Panda or Penguin for example, perhaps it no longer casts a long shadow capable of catching such a diverse group of keywords thus limiting you to only the best keywords (which you're still watching). It will appear as if nothing changed but its very possible, and even likely, that it has.
This is where solid tracking data comes into play. Analytics or log readers are invaluable in spotting changes over time and there are hundreds of good articles on how to break down those reports to look for various signs and signals. Get digging in the data and let us know what it tells you.