CONCAT of DOUBLE with a string cuts off last character sometimes

I found a strange behaviour of DOUBLE-fields.

I have a table with a field duration which is defined as double(4,2)

Then there are the following entries:

8.45

13.55

When I do the following select:

SELECT CONCAT(duration,' hours') FROM table;

I get the following result:

8.45 hours

13.55 hour

as soon as the value exceeds 10, the "s" of hours gets cut off.

changing the field to decimal(4,2) solved the problem for me.