Page 71: "Vehicles always have the Relentless special rule."
Page 41: "Relentles models can shoot with Heavy..."
Page 83: Fast vehicles & Skimmers
Both splintercannons are for gamepurposes on the vehicle like Count Adhemar said.
This means you' ll never use the Assault 4 of the Splinter Cannon on a Venom and always fire at full BS. You can ignore it.
Hope this helps.
Archon of the kabal of The Bleeding Hand.
Member of local Dutch community: http://www.sweetlakesentinels.nl