No. It is, shall we say, fully efficient. The transformer of the label posted has all three sets of windings. The 30? phase shift is because windings are configured delta-wye. Look at the diagram of primary and secondary windings. X0-X1 has same angle as H3-H1, X0-X2 same as H1-H2, and X0-X3 same as H2-H3. As such, each pair of windings are the same phasing.
Now you have to remember that secondary voltage phase relationships are determined by line-to-line voltage, not neutral-to-line voltage. X3-X1 angle is 30? "removed" from that of X0-X1/H3-H1. The same is true of X1-X2 to X0-X2/H1-H2 and X2-X3 to X0-X3/H2-H3.
Let's start with the more basic explanation...
All the load current developed across the open secondary terminals must pass through both secondary windings. Let's say for example you have a balanced 100A 3? load. The current across each winding (1) of a full compliment of secondary windings (3) would be 100A?√3 or 57.74A. However, when you only have two secondary windings in open delta configuration, all 100A must pass through each of the two windings.
The 30? phase shift between winding voltage and winding current is a result of the preceding. Note that when you have full delta secondary windings, the line current splits at the terminal to 57.74A each in my example. Phasing of that current also shifts +/- 30? to align with the voltage across the respective winding. However, when you have an open delta configuration, the current's phasing is not shifted to align with the voltage, and thus the 30? shift... lagging at one open terminal, leading at the other. The windings realize a pf of .87 even when the load pf is 1.