I wouldn't call it weird, Tim. There can be a number of reasons. One is simply that you can't track them down in other years. (The 1841 census is far less user-friendly for our purposes than later years) Another is that 1881 census is freely available (in every sense). If people are Paying as they Go for a census record, they may not be as all-embracing in what records they obtain.
You are of course correct.