The 'site where I found it actually described it as a 3.5 litre but since Jaguar didn't have such an engine they must mean a 3.4
That's just the way they referred to the engine at the Jaguar since, as it was 3,442cc, anyone else would have called it 3½ litre.
Similarly the 2.4 litre at 2483cc would be a 2½ litre anywhere else.
Probably originates in a desire to distinguish the engines from the 2½ and 3½ litre SS pushrod engines (which were 2,663 and 3,485 cc).
Don't let's get involved in the 1½ litre Jaguar (1,776cc) or the 1½ litre MG VA (1,548cc) and 2 litre SA (2,288cc).
I think one can say that displacement is fact and model name is policy