A restatement of the obvious: in deciding whether something has been proved on the balance of probabilities, it is important to have regard to the ability of the party who has the onus to lead evidence on a particular matter.
From Ho v Powell (2001) 51 NSWLR 572; [2001] NSWCA 168; BC200103028 at [16]:
[14] There is a long-standing controversy whether the civil standard of proof requires a numerical probability in excess of 50 per cent (see Davies v Taylor [1974] AC 207 at 219), or belief amounting to reasonable satisfaction (see Briginshaw v Briginshaw (1938) 169 CLR 638 at 642-643). My own opinion is that the resolution of the controversy involves recognition that, in deciding facts according to the civil standard of proof, the court is dealing with two questions: not just what are the probabilities on the limited material which the court has, but also whether that limited material is an appropriate basis on which to reach a reasonable decision. I discussed this in some detail in an article published at (1995) 69 ALJ 731.
[15] In considering the second question, it is important to have regard to the ability of parties, particularly parties bearing the onus of proof, to lead evidence on a particular matter, and the extent to which they have in fact done so: cf 69 ALJ at 732-733, 736, 740. As stated by Lord Mansfield in Blatch v Archer (1774) 1 Cowp 63 at 65 (98 ER 969 at 970):
All evidence is to be weighed according to the proof which it was in the power of one side to have produced, and in the power of the other to have contradicted.