Rabbis who have a vested interest aren't as reliable as those who haven't

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Rabbi Yehuda had placed someone under a ban and that man summoned Rabbi Yehuda to a religious court headed by Rabbi Nachman to protest the ban.
According to Soncino's Translation of  Kiddushin 70b the following conversation took place between Rabbi Nachman and Rabbi Yehuda

‘Why did you place that man under the ban?’
‘Because he abused the Rabbis’ messenger.’ ‘Then you should have punished him [by stripes], for
Rab punished [with stripes] him who abused a messenger of the Rabbis.’ — ‘I dealt with him more
severely.’1 ‘Why did you have it proclaimed that he is a slave?’ He answered: ‘Because he was wont
to call [other] people slaves, and he who declares [others] unfit is [himself] unfit, and never speaks
good [of anyone]; and Samuel said: With his own blemish he stigmatizes [others] as unfit.’ ‘But how
did Samuel say this: only that one must suspect; yet did he say that he is to be [thus] proclaimed?’ At
this stage his opponent said to Rab Judah, ‘You call me a slave, — I who am descended from the
royal house of the Hasmoneans!’ — ‘Thus said Samuel,’ he retorted: ‘Whoever says: "I am
descended from the house of the Hasmoneans is a slave.’2 Said he3 to him, ‘Do you not agree with
what was said by R. Abba in the name of R. Huna in Rab's name: Every scholar who proceeds to
give a ruling:4 if he has stated it before the event, he is heeded; if not, he is not heeded?’5 — ‘But
there is R. Mattenah who supports me,’ he replied. Now, R. Mattenah had not seen Nehardea for
thirteen years, but on that day he visited it. Said he6 to him, ‘Do you remember what Samuel said
when he stood with one foot on the bank and one foot on the bridge?’7 — ‘Thus said Samuel’, he
replied: ‘He who claims, "I am descended from the royal house of the Hasmoneans", is a slave,
because there remained of them only one maiden who ascended a roof, lifted up her voice and cried
out’, "Whoever says I am descended from the house of the Hasmoneans is a slave"; then she fell
from the roof and died.’ So he8 was proclaimed a slave.
Footnotes
(1) Tosaf. in Yeb. 52a suggests that the reason was because he had insulted the Rabbi himself.
(2) Because the dynasty was wiped out by Herod, who, in spite of ascending the throne, was always regarded by the
Jew's as an Idumean slave. He, to exalt his children, called them Hasmoneans, v. B.B. 3b.
(3) Probably R. Nahman.
(4) In his teacher's name.
(5) I.e., when he gives a traditional ruling bearing on his own case, he is believed only if he had stated it before the same
arose.
(6) Rab Judah.
(7) Or ‘on the ferry-boat’.
(8] Rab Judah's litigant.

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To spell the point out  R. Mattenah who had no vested interest in the court case was believed more than Rabbi Yehuda about the status of people claiming they are descendants of the Hasmoneans because when this halacha was brought up by Rabbi Yehuda, he had already a vested interest in this particular ruling.

Let me make it clear that in fact Rabbi Yehuda was telling the halachic truth. Nevertheless because Rabbi Yehuda had a vested interest, Rabbi Nachman wanted to hear the ruling from someone else who had no personal reason to say one way or the other.