Странность MySQL concat () и lower ()

Any idea why this works sensibly*:

mysql> select lower('AB100c');
+-----------------+
| lower('AB100c') |
+-----------------+
| ab100c          |
+-----------------+
1 row in set (0.00 sec)

But this doesn't?

mysql> select lower(concat('A', 'B', 100,'C'));
+----------------------------------+
| lower(concat('A', 'B', 100,'C')) |
+----------------------------------+
| AB100C                           |
+----------------------------------+
1 row in set (0.00 sec)

*sensibly = 'the way I think it should work.'