Confused why so much tissue was taken

I noticed a breast lump a year ago, saw my GP who agreed that I needed to be seen.  Breast clinic did mammogram and ultrasound but said that it was normal breast tissue.  The lump grew and 3 months later my GP agreed that this needed to be checked again.  Again, I was told that there was nothing to worry about and that I should go to a different hospital trust if I was still concerned.  This lump grew and began causing significant pain and limiting my arm movement because it was in the armpit area.  I went to a different hospital 4 months later (thankfully my GP was as concerned as me) and they confirmed that there was a small lesion, possibly a silicon granuloma as I have implants.

So far so good until I had my operation and 6cm of breast tissue was removed.  I had been told that they would only take the small area identified on the scan.

I have been given no clear explanation as to why so much tissue was taken.  There was an 11mm cancerous lump in the middle of this tissue.  They say this was In Situ so not invasive.  So why was the huge amount of surrounding tissue removed?  Nobody seems to know or be willing to say.