Because orb of light ice and fire when combined together are already part of Excalibur (1/2) and it is also required to create sword part of Excalibur (combining with that old gray sword) - and the combination - blue sword - again is (1/2) of Excalibur set (it means that in order to form full set you need to use the orbs again, but they were already used to create a blue sword).
In other words A+B=1/2C 1/2C+A=C => and it means that C= A+B+A=2A+B - but "A" - the orbs - are unique for the campaign (can be obtained only ones) - while the full set (C) requires 2 of A.
Something like that