If the image can be compressed to a point where the image is visually lossless in less than 18mbps then it didn't need to make compremizes and drop information to fit into the 18mbps maximum you set. Why add zero padding if you don't need it (ie bumping the 10mbps to 18)?
As Jim asks, what are the export sizes? - 480p is a lot smaller than 1080p and thus takes 4 times less to describe.
Also, what is the material? If it's mostly static images then you definitely don't need 18mpbs to describe it. For example, last week I rendered out a screen capture of a presentation at 1080p and was able to do it lossless at 1mbps since almost nothing changed except the mouse moving and windows popping up occasionally. In that case I set the max to 4mbps but afterwards (in MediaInfo) saw that it only needed about 1mbps.
