Inviting an allegation that I'm arrogant again, but the textbook is wrong if it does say that. Really is as simple as that. If you show me exactly what it says I can explain but there is only one standard of proof in civil cases: the balance of probabilities.
The more serious the allegation, the less likely it is to have occurred. But this does not mean that where a serious allegation is in issue, the standard of
proof required is higher. It means only that the inherent probability or improbability of an event is itself a matter to be taken into account when weighing the probabilities and deciding whether, on balance, the event occurred see H (Minors) [1996] A.C. 563 at 586 per Lord Nicholls.
It is well discussed in this case
https://www.bailii.org/cgi-bin/form...Bank+St+Petersburg+PJSC+v+Vitaly+Arkhangelsky