A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
To explain.... The format buttons (formatting, in general) affects only the apppearance of a value. It does not change the value itself.
12.34% is the decimal value 0.1234. 1234% is the decimal value 12.34. So if we enter the decimal value 12, it is indeed 1200%.
If you want to enter 12 and have Excel interpret it as 12%, select the option "Enable automatic percent entry". I find it by clicking File > Options > Advanced. The navigation might be different for your version of Excel.
With that option set, for any cell that is already formatted as Percentage, when we type 12, Excel will interpret that as 12%, which is the decimal value 0.12.
But IMHO, it is easier to type 12% (i.e. with the "%" suffix).
If you have already entered decimal values that you want to be treated as percentages, enter 100 into a cell and copy that cell, then right-click the range of decimal values, and click Paste Special > Divide. You might also need to apply the Percentage format that you desire.