Can sources be trusted?

Today I discovered an interesting situation whilst conducting my research. I had three secondary sources that all provided different figures for Tulk's (SLVs first Librarian) intial salary. What made it more interesting was two sources were produced by the same person. And one of those was the entry in the Australian Dictionary of Biography (an authorative reference).

Here are the three:

"The salary provided on the estimates was ₤210 for ten months, and the Trustees asked that this amount be paid to Mr Tulk rateably for the eight remaining months of the year." (Armstrong 1906, p.6)

"The first Librarian was, however, Augustus Henry Tulk, chosen from among 48 candidates and appointed in May, 1856, at a salary of about ₤250 a year. (McCallum 1959, p.37)

“...he took up his post on 5 May 1856 at a salary of ₤600.” (McCallum 1969, p.308)

What was Augustus Tulk initially paid? I had to find some primary sources to substantiate any of the claims.

I found two entries in The Argus indexes for Tulk's salary. The first entry was "Salary of librarian ₤210 p.a." 17-3-56 (5). (This was just months before Tulk was appointed). I was not able to locate the article online. The Newspaper Digitisation Project has every issue of The Argus for that month, but the one I wanted (even the NDP can't be relied upon!). However, the index entry gives the fact we need: ₤210. This is the same amount Armstrong details, and close to the figure McCallum's first source details. One cannot say with certainity that Tulk was paid the same as what was initially allocated. However, it is more likely than not.

The second entry was a letter in The Argus dated 22nd June 1861. It states: " whilst for the librarian of the Public Library, the sum set down last year was £600 only." So in 1860 Tulk was paid £600. This is the same amount that McCallum's ABD entry attributes to Tulk's first year salary. This is possibly where the confusion arose. Although there is an error that McCallum performed by not checking his own work for the fact.

The Australian Dictionary of Biography is an authorative reference work that is highly regarded and trusted. Yet an error exists within one of its entries. Can any work truly claim absolute accuracy? If not, what does this mean for research? Multiple sources must be found to substantiate the same thing. Leading to duplication, which although time consuming is necessary. Although what even all the sources bear the same fault?How can the truth be found? Can sources be trusted?

Bibliography:
1856, The Argus, 17 March, p.5.

1861, The Argus, 22 June, p.7

Armstrong, E.L.T. 1906 The book of the Public Library, Museums, and National Gallery of Victoria, 1856-1906, Trustees of the Public Library, Museums, and National Gallery of Victoria, Melbourne.

McCallum, C.A. 1956, The Public Library of Victoria, 1856-1956, [The Library], Melbourne.

McCallum, C.A. 1969, 'Tulk, Augustus Henry (1810-1873)', Australian dictionary of biography, vol. 6, Melbourne University Press, Carlton, Vic., pp. 308-309.