To fix this, you can update the format of the affected columns in Excel with these steps:

1. Highlight the column
2. Right-click the column and select Format Cells
3. On the Number tab, select Special
4. Select Zip Code as the Type
5. Click OK. 

This article also references this issue and additional options for retaining leading zeros in Excel: https://support.office.com/en-us/article/Keeping-leading-zeros-and-large-numbers-1bf7b935-36e1-4985-842f-5dfa51f85fe7?ui=en-US&rs=en-US&ad=US