Only a guess (not having worked their a while) but:
1) Standard join out of 'TG' is WOTAN (not MALBY), where FL70 is outside CAS - hence higher level (refer to UK AIP EGTG AD 2.22, also included in AERADs / Jepps )
2) Joining FL70 at Malby puts you in the teeth of 'DL' departures on their SID, hence higher level requirement (and different join point referred to above).