This is a common problem with Excel. One thing I've found is that if you change the row sample size that Excel uses (by way of the Access Jet driver) to determine the data type in your Excel document, you can get more accurate results. See the posting in the following link for this registry hack:

http://msmvps.com/blogs/nickwienholt/archive/2006/03/15/86379.aspx

hth,

Tim