The key Talmudic sugyot on bitul discuss the general rules of nullification (1-in-60 for most items) and explicitly note that terumah requires 1-in-100 (101 total), reasoning that its sanctity — as something 'set apart' for the priest — gives it special status that resists ordinary bitul.
וְקָתָנֵי סֵיפָא לְהָקֵל וּלְהַחְמִיר מִין וְשֶׁאֵינוֹ מִינוֹ, כֵּיצַד? גְּרִיסִין שֶׁנִּתְבַּשְּׁלוּ עִם הָעֲדָשִׁים, אִם יֵשׁ בָּהֶם בְּנוֹתֵן טַעַם, בֵּין יֵשׁ בָּהֶן לְהַעֲלוֹת בְּמֵאָה וְאֶחָד, בֵּין אֵין בָּהֶן לְהַעֲלוֹת בְּמֵאָה וְאֶחָד – אָסוּר. אֵין בָּהֶן בְּנוֹתֵן טַעַם, בֵּין שֶׁיֵּשׁ בָּהֶן לְהַעֲלוֹת בְּמֵאָה וְאֶחָד, בֵּין אֵין בָּהֶן לְהַעֲלוֹת בְּמֵאָה וְאֶחָד – מוּתָּר. אֵין בָּהֶן לְהַעֲלוֹת בְּמֵאָה וְאֶחָד, אֶלָּא בְּמַאי? לָאו בְּשִׁשִּׁים?
And it is taught in the latter clause, i.e., in the following mishna (Orla 2:7): When it is stated that the halakha is to be lenient and to be stringent if he mixed one type of teruma with non-sacred food not of its type, how is this accomplished? For example, in a case of split beans of teruma that were cooked with non-sacred lentils, if there are enough split beans relative to the lentils to impart flavor to the lentils, then regardless of whether there are enough lentils to neutralize the split beans in one hundred and one times their volume, i.e., the volume of the lentils is one hundred and one times the volume of the split beans, or whether there are not enough lentils to neutralize the split beans in one hundred and one times their volume, the entire mixture is forbidden to a non-priest. Conversely, if there are not enough split beans of teruma to impart flavor to the lentils, then regardless of whether there are enough lentils to neutralize the split beans in one hundred and one times their volume, or whether there are not enough lentils to neutralize the split beans in one hundred and one times their volume, the mixture is permitted even to a non-priest. Abaye commented: When the mishna says that there are not enough lentils to neutralize the split beans in one hundred and one times their volume, and nevertheless if the split beans did not impart flavor to the lentils, the mixture is permitted, in what amount of lentils are the split beans nullified? Is it not that the split beans are permitted because they are mixed with sixty times their volume of non-sacred lentils? This contradicts the opinion of Rabbi Shmuel bar Rav Yitzḥak.