Normally in Excel if you convert a cell to a percentage it moves the decimal 2 spots to the right. Once it is formatted you can just type in the number and it accepts it as the percent. It sounds like the values in your percentage column are entered as whole numbers, so when you are formatting them as a percent it looks like they are multiplied by 100 (the decimal point is moved 2 places to the right making 25 into 2500)
Wait a second! Are you formatting values that
are greater than 1 as a percent value? Remember,
.01 is 1%. If you enter 1 and format it as a
percent you get 100%; however, internally it's
still stored as 1.
A technicality, but an common misconception...
Formatting doesn't change the internal representation
of a value; therefore, the decimal isn't moved
by the formatting itself. If you enter 10% into
a cell that formatted as General. Two things
happen. The % percent operation alters the
value from 10 to 0.1. Next, Excel recognizes
that your entering a percent value and applies
the Percentage format which affects the manner
in which the value is displayed.