AFAIK/R, this generally happens because of report design. It has it's own grid-guidlines and snap-togrid thing. At times, we adjust controls based on our choice and it does not fit in with the same grid-snap as expected. These small adjustments while exporting results in small columns.
Let me see, if I can provide you a link with proof.
UPDATE:
This link[
^] says something similar but not for SSRS. But the reason/logic goes the same way.