Why the TxnStore Migration will not handle any delta migration of transactional data?

the TxnStore Migration will not handle any delta migration of transactional data

TxnStore data migration happens as part of the reporting migration as this step can be carried at the end of migration, so that there will not be any delta migration requirement for transactional data.
TrxStore Migration (Step 2 in Migration steps) will perform only the migration of robot data.