Studio Sends: -16.6 dB lower than mains?

In Nuendo 4, I need to have a signal from a Studio Send to be exactly the same level as the Signal going to the LFE channel of a 5.1 mix. I use an RME Multiface and Totalmix is the hardware mixer it comes with.

A signal enters a Mono Group Channel peaking at -0.8 dB on the Group Channel Meter.
This Group Fader is at 0.00.
I assign the Group to LFE out of the 5.1 Mains.
The Group Channel and the Control Room Mains Meters correspond 1:1 and the Totalmix meter is only off by 0.3 dB.

I then assign this signal to a mono Studio Send on this Group Channel.
With Studio Send gain at 0.00 and the Studio Fader at 0.00:
the Studio Send meter reads -16.6 dB lower than the signal coming out of the LFE channel, and the TotalMix level for this channel peaks at same which confirms the difference as actual.

If I put a gain cell insert (Sonalksis Free-G) and boost the signal on the Studio Send channel by 16.6 dB, both the Nuendo meters and Totalmix show they are now equal.

I checked to make sure there aren’t any other paths to these outputs (sends. studio sends, & track assignments).

I confirmed this using the Test Tone generator from a mono audio track: there is a 16.6 dB difference in the level that goes to a Studio Send vs a direct output.

Questions: Why? Or am I missing something?