Number Formats Automatically Change

In the attached workbook I have done nothing: no cell formatting or anything. I think I may have accidentally changed something in Excel to do this; but when I open a new workbook, and type a general number, the format changes to where a decimal is added, or the number changes completely. Exempli gratia, I type 666666; the cell changes it to 666.66; I type 1973; the cell changes to 19.73. Remember: I just opened the workbook and typed numbers. I haven’t formatted a cell, and then typed it. It isn’t reserved to one workbook. It does it to all workbooks of mine. I have Excel 2016 on a Mac.
Please help me. Thank you.