Just speculating here...
I suspect they meant that they couldn't be sure they removed every bit of the polyp's tissue because there were extensions infiltrating below the surface of the colon's lining. If there's no re-growth it was more likely that they did get rid of it. Adenomas are "neoplastic" (a neoplasm is an abnormal growth of new cells as opposed to an inflammatory response by existing cells) Neoplastic polyps
can eventually become cancerous, which is why they want to re-check the ones they removed. If they can't remove the entire polyp its possible for it to form again, and that new growth might not be benign. Some info I found about various types of polyps that might be of interest:
https://www.mayoclinic.org/diseases-...s/syc-20352875
https://librepathology.org/wiki/Gast...l_tract_polyps