What is the difference btw. M_TABLES and TABLES view

I looked into the definition of these two HANA views which are a bit complex but look similar, BUT from a 'documentation' point of view they are equal (=information on row and column tables).

Interestingly, there is a difference in the results I get when I query these tables. So, in order to find the differences I ran the following two queries with surprising results:

select schema_name, table_name from tables

minus

select schema_name, table_name from m_tables;

This should give me tables from system view "TABLES" which are NOT in system view "M_TABLES". And there are some.

Vice versa, when running the follwing query:

select schema_name, table_name from m_tables

minus

select schema_name, table_name from tables;

gives a different result set.

Why??