You should set the format in the table to the number of decimals that you desire (make sure that the number is a double or single, which is a real number (decimal)). Even if your form is set to a decimal format, if you have the field set as an integer, it will be saved as an integer (with rounding).
If you do that and still have problems, check the regional settings on the computer and make sure that the number format is set right.