In more recent time frames such as after 1950, it seems to be more likely that families, and thus also those who record them, will go Sr. Jr, III, IV etc.
But in older times such as early census records, John Brown Sr. and John Brown Jr might not even be related, as Melanie pointed out. It would just indicate that John Brown Sr is older than John Brown Jr. However, by the next enumeration 10 years later, John Brown Sr may have passed away. I have definitely seen situations where the person called Jr might now be Sr, if a younger 'John Brown' has come of age to be enumerated. I haven't seen a situation where a newcomer to town was older and took the vacated Sr designation, but it is possible. In the earlier times, the Sr and Jr just helped sort out residents of the same name. It might coincidentally place them in their family trees as well, but that was not the intention on the census.
Memorial stones might be another matter -- i haven't really been noticing.
Cheers
Shirlea