Yes, all sources, of every possible kind are subject to errors. Nothing is immune. Census records - these people went door to door and often if someone wasn't home, they might have asked a neighbor for information. Or, maybe a kid was the only one around to answer the questions. Of course, I have no idea about your own research, but maybe, just maybe you have found the similar but the wrong people in that 1900 census. There are a lot of differences, as you say. Alternate spellings/bad spellings often lead the search engine not to display the intended results.
Edit - I think I just found who you were talking about in Teller County, CO. The census says William was a clerk in a General Store and his brother Thomas was a civil engineer. Curious, it says William came to America in 1862 and both their fathers were born in Ireland and mother in Canada