Curious results from pg_stat_user_indexes

**PostgreSQL 12.8, PostGIS 3.1.4** Running select * from pg_stat_user_indexes, there are some rows that have 0 for idx_scan, and a >0 result for idx_tup_read. The indexes that match this condition are all simple SP-Gist indexes, i.e.,

CREATE INDEX ... USING spgist(geometry)

The definition for idx_tup_read is "Number of index entries returned by scans on this index." If idx_scan is 0, I would expect "scans on this index" to be 0 as well. So how can idx_tup_read have a result greater than 0, if idx_scan is 0?