This is because the information inside the spreadsheet was not over-written with new data - instead if was deleted. So when re-imported into the system it did not over-write the original data that was recorded against the contacts.
If you are wanting to update information against contacts in bulk then the best way to do this is to export out the data from the CRM system, delete any column you do not wish to update.
Please Remember: ALWAYS keep in the first column which it titled ICUniqueID as this is the code which links the exported contacts back into their correct contact records inside the CRM system.
To find out more about exporting data to update and then re-import back into your CRM account please click here.