Index On Mysql Column Has Lower Cardinality Than Count Distinct?

I have a varchar(50) field named token. When I do count(distinct token) from table I get ~ 400k. However, if I do create index idx on table (token) The cardinality is only 200

Solution 1:

The cardinality is an estimate of the number of unique values in the index. According to the documentation:

Cardinality is counted based on statistics stored as integers, so the value is not necessarily exact even for small tables.