In my research about
these diseases I have read that the ileum is involved in the majority of Crohn's patients, and that when healing UC can look patchy. That last part made sense to me because when I have a scrape on my skin it doesn't always heal evenly -- some spots heal faster than others -- so I figured the colon could be the same way.
"In addition, patchiness of the colonic mucosa suggestive of skip lesions may occur during the treatment phase of ulcerative colitis, leading one to question the diagnosis. These patchy areas may be seen endoscopically in as many as 38% of patients with ulcerative colitis who undergo medical therapy. Rectal sparing may also occur at some point during medical treatment of ulcerative colitis in as many as 44% of cases."
emedicine.medscape.com/article/183084-differential