If I claim X, you get straightforward evidence in favour of at least two (correlated but distinct) hypotheses:
My claiming X is evidence for X's objective truth insofar as I tend to claim true things more often than false things. If I add details, or substantiating observations, to my claim, the claim is stronger evidence, insofar as I could come up with those kinds of details for true things more relaibly than false things.
When I argue — not carefully and seriously, but in the violent manner of a quarrel or dispute — I follow the apparent convention to directly and completely oppose the opponent. At the start, I state actual beliefs (X), which only partly contradict the opponents' (Z). As a dispute progresses, what I state (and perhaps what the opponent states) converges to the exact opposite (Y) of the opponent's (respectively, my) claim.
If this does not happen, we would find some agreement (X ≈ Z) — there is always some agreement in reality — and calm down. In that case, what you see would not be an argument, and remarks here would not apply.
Arguments expect details to support claims more than average conversation, so when arguing, I am likely to add detail to Y. When those details are true, my claim of Y is stronger evidence for Y's objective truth (than a casual mention outside an argument). X (my underlying belief) may nevertheless be more accurate than Y, having more supporting details than Y, which I don't bother to reveal. If it weren't, and I knew that, I'd be inconsistent.
An elaborately-supported claim is not necessarily a claim like Y in this pattern. The basis for this pattern is adversarial two-way assertion. Elaborately supporting a claim, but peacefully, in isolation (as in most of my essays), is not subject to the effect described here.
Independent of the relative truth of X and Y, you should not conclude from my quarreling in favour of Y that I believe Y. The accurate conclusion is that I believe X, but you may have no way of knowing X from the quarrel.
My casual remarks expose my beliefs more accurately than enthusiastic shouting.