Why were the Triarii differently equipped?

[Urselius]

It was not necessarily an either/or situation. States could, and often did keep arsenals of weapons and armour, often centrally stored, and expect most of the soldiery to provide their own equipment. We know this is true of the Komnenian period Byzantine army. Soldiers were expected to equip themselves out of their pay and/or grants of taxes from imperial land (pronoia), however, in a crisis the emperor Manuel I is recorded as issuing mailcoats to soldiers from an arsenal in Constantinople.