What does SPLADE change compared to a traditional BM25...

While SPLADE maintains the scoring form of a traditional inverted index (s(q, d) = ∑ wq_j * wd_j), it fundamentally changes how weights are calculated. Instead of relying on term frequency, both query and document weight functions are replaced by an MLM head over the full 30,522 vocabulary entries. The trade-off for this improved semantic matching is an increase in posting-list length, as more terms are assigned non-zero weights.