Why do models struggle with arithmetic and with spelling questions?

An answer that works

Both follow from the segmentation. A number may be one token or three depending on whether that digit sequence was frequent enough to merge, so digits that should be treated positionally arrive in inconsistent groupings — several tokenizers now force digits into fixed groups for exactly this reason. Spelling questions fail for the same reason from the other direction: a word that arrives as a single token gives the model no direct view of its letters.