cDNA sequence differences

[Leo]

  My question may be a simple one but it is important
 to my research and I will be grateful if someone can
 provide me some information.

  Recently we have isolated a cDNA clone from a Stratagene
  human liver cDNA library. The deduced amino acid sequence of
  the gene product matched very well with that of the enzyme
  we are interested in. Only two substitutions out of 400 or so
  amino acid residues were noted.  The nucleotide sequence of
  the cDNA also matched several human EST files in GenBank.
  However, the homology between the cDNA sequence and those of the
  human EST files is only 93%.
 My questions are:
  1. Is it possible that the Stratagene cDNA library is not human origin ?
  2. Did anyone has a similar experience ?
  3. How reliable are the commercial cDNA libraries (in terms of
     their source) ? For an example, I have heard a laboratory has
     isolated a bacterial gene from a human cDNA library.
  4. How different should I expect to see when comparing cDNA sequences
     of a housekeeping enzyme derived from two different individuals
     (of two different races) ?

  Thank a lot for your kind response.