I'm gonna make a guess that it does record analog at 24-bit but only at the 48kHz sampling rate. 16-bit analog recording is supported at all the rates listed above. Perhaps the reason for this is that the 24-bit ADC output is somehow routed to the S/PDIF input which makes it different and more limited than the 16-bit ADC.24-bit Analog-to-Digital conversion of analog inputs at 48kHz sample rate
24-bit Digital-to-Analog conversion of digital sources at 48kHz to analog 5.1 speaker output
16-bit recording with sampling rates of 8, 11.025, 16, 22.05, 24, 32, 44.1, and 48kHz
Supports Sony® / Philips® Digital Interface (SPDIF) format input signal of up to 24-bit/96kHz quality