storage/xtradb/dict/dict0dict.c:dict_stats_update_transient (ha_innobase::info_low 10.2) iterates over all indexes of a table to update the statistics.

Inside this loop the call to btr_estimate_number_of_different_key_vals fetches a different set of random pages for updating each index.

Since the pages are at random, taking the largest n_sample_pages of pages based on index size and using the same set of pages to calculating the statistics for all indexes will result in less pages retrieved and more accurate statistics (for indexes per table > 1).