Many factors can influence the yield. It depends on sequence
composition and oligo length.
The main reason why you might see different yields for the same
oligo (same sequence, same length), might be due to the column
loading with CPG (controlled pore glass) material, used to initiate
the synthesis. The minimum loading corresponds to the synthesis
scale but fluctuation of the CPG amount and CPG loading can occur,
which can lead to varying yields.