Many spreadsheet editors, including Excel, Numbers and Google Sheets, will default to excluding leading zeroes from cells with a data format of Number.
As such, it is possible that when you open a spreadsheet downloaded from Geocodio, that appended fields that contain numbers with leading zeroes (such as Zip Codes) may be returned without them.
Our API returns the numbers in correct formatting, but Excel's default settings could render that data in the spreadsheet incorrectly. The best way to address this problem in Excel is to change the default data format of the effected column to Text or to create a custom data format for that column.
Here are some resources to help you create a custom data format: