Excel Keeps Changing Data By Itself

I use Excel at work and every 2 weeks for payroll I copy the Excel file, rename it with the current pay period's date and then clear out all the hours and other values and re-enter new hours, etc. Last pay period, payroll pointed out a column of cells that all the formulas were off by one row so I fixed it. I just happened to notice those formulas have somehow reverted back to what they were before. now, how is that possible? I took a screenshot of the two sheets side by side. On the left, the original file and the right is the copy.

This happens ALL the time and it's very hard to catch half the time. I've had to drive in to work on my day off more times than I care to count to fix these glitches that I thought were my mistakes for a year until I saw it happen in real time one day and realized the software had put a whole column of old deleted data back in.

Is this a common Excel problem? How does it still have data from weeks or months back? How would the original file have one set of data and copying the file and opening it up, cause it to have a different set of data in it.

Just now I copied the file (right click, copy, paste, rename file to current pay period date range) and I opened it and noticed that a person who I removed last pay period is back and the new employee I added is gone. Just to verify I'm not going crazy, I opened the old version of the file that I just copied, side by side with the new copy and sure enough, the old copy has the new employee listed and the other employee removed.

How is this possible? A copied file ought to be an exact replica of the original.

Then I copied it again and it was fixed in the second copy. There's some kind of very strange glitch going on in Excel.

Someone mentioned one drive or shared drive. Yes, it is on the network drive of the LAN here at work.

Excel Version 9.0