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Turkish Airlines applies the MSC (Most Significant Carrier) rule. We'll come to that in a minute. Firstly it's worth noting that their 'official' baggage allowance is on their website. So that's the first point of call. It lists economy class limits at 20kg.
However, all over that page are exceptions. Example:
Free baggage allowance from Germany to Turkey and Beyond Turkey for Economy Class is 30 kg and for Business Class 40 kg.
So there's already evidence of exceptions, however, none that cover your instance.
Then the MSC rule comes in.
> MSC (Most Significant Carrier)
If another airline is involved in your whole journey; the most significant carrierβs (MSC) free baggage allowance rule will be applied. MSC is assigned according to which airline operates the most significant and/or longest part of the journey. If Turkish Airline is MSC in your journey then Turkish Airlineβs free baggage allowance will apply, if another airline is MSC in your journey than the free baggage allowance of that airline will apply.
> Please pay attention that the free baggage allowance is shown on your ticket.
So it's quite possible that your airline is code-sharing with another airline that has a higher baggage allowance. As a result (see last line above), the final point of call is the value on your ticket. It's the end result of agreements between Turkish's limits, exceptions, and codeshare MSC agreements.
As for if any disagreement comes up about differences between the website and the ticket, it's fairly easy to point to the line on the website that says [the allowance] 'is shown on your ticket' :)