Question about get_next_slot_proofs incoming block height

Why the block_no passed in get next slot proofs needs to add the block_count, Instead of using block_no in initial_db directly

let block_no = initial_db.db.borrow_db().block_no + block_count - 1;

which means that this is beyond the height of storage orphan.
If the slot will be modified in subsequent blocks, will this proof be invalid?