Several things, There's nothing to stop you from setting the aperture between stops. There just aren't clicks and it's not precise. It is "close enough for government work" though.
The factors are suggestions from the manufacturer. just like film you may find you need to apply a little "windage"
Often reading through the filter will not give what you need because of the spectral response of the metering cell.
Try a few test exposures using the ttl method and the using the marked recommendation.