modulation – Why does NB-IoT use standard QPSK in multi-tone transmission, when pi/4 QPSK is implemented for single-tone?

I’ve been studying the modulation schemes used in NB-IoT communication, with a particular focus on energy efficiency. I’ve noticed that QPSK (Quadrature Phase Shift Keying) is used in multi-tone transmission, while π/4-QPSK is often used in single-tone transmission (scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/application_notes/1ma296/1MA296_2e_NB-IoT_Measurements.pdf or digibug.ugr.es/bitstream/handle/10481/56821/67983.pdf table 3.7 and 3.8). I’m curious about the reasons behind this choice of modulation schemes.

• Why is QPSK preferred for multi-tone transmission in NB-IoT,
  especially considering that π/4-QPSK is used for single-tone
  transmission?
• What are the advantages and trade-offs of using QPSK
  over π/4-QPSK in multi-tone transmission for NB-IoT devices?
• Are there specific technical or practical considerations (energy efficiency, throughput or coverage for example) that make QPSK a more suitable choice than π/4-QPSK, particularly in good coverage conditions?

I would appreciate any insights or explanations regarding the modulation choices for these two NB-IoT transmission scenarios. Thank you!