conflicting reports about vascular invasion

Hi,
I had a core needle biopsy first that indicated that "vascular invasion was present, in a few small angiolymphatic spaces."  However my mastectomy pathology report stated that "vascular invasion was not identified."  Why would these two reports have different findings?  

[BJSaws]

A biopsy is just a very small sample.  The pathology from the mastectomy has the entire tumor to work with.  I would go with the report from the mastectomy.  My biopsy said my tumor was adenocarcinoma, but the pathology from the mastectomy identified the tumor as medullary carcinoma.  I was told that it is not unusual for the biopsy and mastectomy pathologies to be different and you should go with the one that takes the larger amount of tissue (the mastectomy).  I'm not a medical professional just a person who experienced conflicting pathology reports.