It is documented in AutoCAD Help (see 'Use Windows Cut, Copy, and Paste' in UG) but still many AutoCAD users 'misuse' this functionality -- clipboard Copy/Paste is not suitable for copying entities from/to AutoCAD drawings, as you will loose precision (plus extended entity data, reactors, dbLinks, etc.). Windows Clipboard (WMF) is intended for transferring data to/from other Windows applications.
The differences in simply copied/pasted coordinates are on about the 9th decimal place so it may not come into your attention immediately but the precision lost is significant (influences dims, osnaps, etc.).
In the sample drawing below, the original (red) rectangle [0,0] - [1000000.1,1000000.1] was copied using Copy/Paste (Ctrl+C/Ctrl+V), then the _COPY command and then using _COPYBASE - Copy with basepoint (and then offset by [1,1] to make it readable). Only the Copy/Paste method distorts the vertex coordinates, making 1000000.0995 from 1000000.1.
The 'Copy with basepoint' function sets the exact coordinates and keeps full AutoCAD precision.
Clipboard-instable.dwg (30kB, right-click/Save to download)
copypaste.scr (script file demonstrating the problem; use on an empty drawing)
(Precision lost depends on your Windows version, your MS Office version and applied MS Office Service Packs.)