Oh, one more thing...I am only 35 (female) and have this happening, so keep in mind that just b/c they say '50' (cancerous polyps) doesn't discriminate on age; Just want you to be aware of that.
I too have had precancerous polyps removed (years ago and just recently) but my doctor says to come back much sooner than 10 years when a 'pre cancerous polyp' has been present. I think it's 2 years, I wouldn't wait that long or at least ask another doctor's opinion on that (10 years is an awfully long time to wait for another screening if history of precancerous polyps).
I had three removed in 2007 and two weeks ago had a sigmoidoscopy (like a colonoscopy but only scopes the lower bowel) and found a new precancerous polyp. So it doesn't always take long to develop.
I would strongly suggest asking another GI specialist's opinion and switching doctors; that just doesn't sound right to me.
If you use the google search engine and enter the words 'colon polyps' you'll pull up an overview by the Mayo Clinic. It will give you a good idea of the 'why and wherefore's.'