Speeds can vary due to a number of reasons, not least being the state of the drive from which you are copying data to the pen drive.
For example, if the source drive is heavily fragmented, that will affect transfer speed since the target drive cannot write any faster than the read speed of the source drive. It's worth checking fragmentation levels and defragging if it's higher than 10%.
Just a quick bit of advice - - don't choose a post as "Best Answer" unless it fixes your problem, it's not appropriate.