This is a known issue caused by differences in the precision of Excel and FileMaker Floating point values. You can use an auto-entered calculation during import or replace fields immediately after import with the round function to make the needed adjustment to your values.
For More Information see: Import from Excel decimal errors
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip