SQL Server Compression Estimate Is Significantly Lower When Estimating Current Compression Type
Check out this link - http://www.bobpusateri.com/archive/2012/04/compression-estimates-that-dont-make-sense/


  BOL states that “If the requested compression setting is same as the current compression setting, the stored procedure will return the estimated size with no data fragmentation and using the existing fill factor.”


The above link does point to a Microsoft Connect item that was opened that seemed 'somewhat' similar, but not exactly like your issue.  Microsoft close it indicating no fix would be forthcoming.