A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
I know this is an old thread, but the problem remains nonetheless; and another quick solution seems warranted.
One thing to check for is hidden values (blank/string) that precede the numeric value. Hidden values such as tabs, spaces, and other "clear" characters might not be visually evident, but could still exist. Those would force Excel to see the cell as text, regardless of whether you change it to general or numeric.
Click within the cell, and go all the way to the left and right. You might be shocked by some hidden values. These probably came from copy/paste between formats/encodings
To resolve, use the "replace all" function to strip out any instances you find, such as spaces or tabs etc.
Worked for me, after pulling out my hair after changing to General or Numeric didn't work.