I assume that the figures they are giving is for the total over practise and race week?
I guess the difference this year on the Manannan will be the extra mez level.
However for a casual observation, I have been coming over on the Friday ( practice week) night boat from Liverpool for a number of years now. If you went back about 3 years, on that sailing, the majority of the lower car deck was 99% motorbikes, with the odd van, tucked into the odd space. In the last 2 years at least, the whole centre section of the lower deck had no bikes, and was filled with cars and vans, so my observation was there has been less bikes on what is historically the busiest day for trafic going to the Island.