MySql decimal types are a little bit more complicated than just left-of and right-of the decimal point.
The first argument is precision, which is the number of total digits. The second argument is scale which is the maximum number of digits to the right of the decimal point.
Thus, (4,2) can be anything from -99.99 to 99.99.
As for why you’re getting 99.99 instead of the desired 3.80, the value you’re inserting must be interpreted as larger than 99.99, so the max value is used. Maybe you could post the code that you are using to insert or update the table.
Edit
Corrected a misunderstanding of the usage of scale and precision, per http://dev.mysql.com/doc/refman/5.0/en/numeric-types.html.