Unexpected changes by the default WordPress editor

My site was built by a web designer (he left 2 years ago), and I maintain it. He was using the plugin CKEditor to edit pages, which I deactivated in favor of the default WordPress editor. I edit individual pages now, and sometimes I try to change the HTML directly. Occasionally, when I update a page then go back and look at the HTML, the spacing and tag positions are slightly different than what I saved! I can’t explain it, except for the possibility that my HTML was wrong and the default editor tried to fix it. Could this be a different problem, and how would I test for that? Thanks