There's a difference between how FileMaker and Excel handle floating point numbers. Both use an internal "binary scientific notation format" but with a different number of digits in the mantissa if I've decoded the info from Filemaker correctly.
The best solution I can recommend is to follow up your import with a Replace Field Contents operation that rounds the contents of this field slightly. This can be done manually or in a script.
You can use the calculation option of Replace Fields and use something like: Round ( YourField ; 11 ) to clean them up.
While FileMaker argues that this isn't really a bug...
For More Information see: Import from Excel decimal errors
This is one of many acknowledged issues that can be found in the Known Bug List here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html