>>101611415Because Japanese kanji is ridiculous and not even the Japanese can read them. Normal kanji can change readings depending on the word or if they're in names and there can be hundreds or thousands of names that include that kanji.
Think of it this way: if you come across a new kanji, how are you supposed to look up the way to pronounce it? You can't spell it out. You can't even look it up unless you're on a computer and can copy-paste it into a search engine.
So Subaru read the "tani" (่ฐท) character in Tanigo (่ฐท้ท) as "ya" because she didn't recognize his name right away and she thought it changed readings.
The funny part is that the kanji ่ฐท is usually read as "tani" in most words. It just means valley. ่ฐท้ is "tanima" for example and means valley gorge, as well as being the japanese word for a woman's cleavage.
The other reading "ya" is way less common in regular words, as it's mostly only used in ่ฐทๅฐ which is read "yachi" and it means swamp/marsh.
However, there are hundreds of names where it's read as either "ya" or "tani" with "tani" seemingly being a bit more common. This probably means she knows someone in real life with a surname including that kanji who pronounces it with "ya".
