Reason for slowing down on managed migration is ‘in order to’ reduce costs of TP says OBR

Yesterday’s Economic and fiscal outlook finds that (paragraph 4.85) -

... the Government has reduced volumes to well below DWP’s operational capacity in order to reduce the costs associated with transitional protection for claimants who would otherwise lose out relative to their legacy benefit claim

Not quite the way the government has presented it…

The budget documents last year indicated significant savings were expected due to the slowing down of managed migration. I made a FoI request asking for the analysis behind the figures. The Treasury confirmed they had the information but refused to release the information on public interest grounds.
See here: https://www.whatdotheyknow.com/request/budget_2018_universal_credit_rol#incoming-1319777