Why would one not be surprised that when students were asked how many minutes they studied on a typical weeknight, most responses are in multiples of ten minutes????
This question seems to be more about psychology than statistics.
Most people don't recall study time intervals very accurately, so they tend to recall them in terms of one significant figure. Ten is the basis of our numbering system. Most do not study more than 90 minutes, probably. Thinking in terms of fractions of an hour is awkward.