[BioC] difference between hgu95av2probe library and pm matrix

[M.Ruschhaupt at dkfz-heidelberg.de]

Dear all, 
  
I have just started to work with R and Bioconductor. Now I have a question about 
the hgu95av2probe library and also the hgu95probe library. Does anybody know, why 
not all probes are listed there.   
  
For example the pefect match matrix in the Dilution affy batch object from the 
affycomp package is an matrix with 201800 rows (probs) but the hgu95av2probe object only 
has 199084 rows. So there is a difference of about 2700 probes. Is their sequence unknown or 
why are these probes not listed in hgu95av2probe ? 
  
Thank you 
M. Ruschhaupt