My understanding of travellers and the census, is that until 1871, travellers tended not to be included, as the census focused on those living in houses. In 1841 the census was taken on 6 June, in 1851 it was 30 March, and in 1861 it was 7th April. Being early in the year, it could be before they started travelling, having spent winter in a house. Another issue could be illness, especially a child, where some remained in a house while the rest travelled. Finally, they tended to have large families and then not all travelled. Within your grouping, some could be land-based, but they would be welcoming to their traveller relatives. Finding earlier travellers can be very difficult.