coordinate system - Importing .csv to QGIS gives CRS undefined (but not always)

I wonder why some .csv files when imported (with long and lat) can not set a proper CRS system and other do (CRS undefined). For example:

1. If I export my data in csv with Microsoft Excel it does not set the CRS and can not be fixed.


2. when I export with Google Sheets it does work.

What is the problem in this case? I am trying to export data with pandas python but can not figure out this problem? I am using QGIS, 2.12.1-Lyon, on MAC.