There is a 6dB gain reduction going through the SampleCell TDM Plug-In. The reason for this attenuation is as follows:
SCII with a TDM piggyback contains a DSP that is responsible for getting the output of SCII onto TDM and for running the "SampleCell Plug-In" that gets the data into the Pro Tools environment. The SCII Plug-In functionality is limited by this DSP (it has to take care of a bunch of filtering, and doesn't have variable gain capability). Many people use Instruments with SCII that contain samples that have peaks that reach "full code," and it is common to play multiple samples simultaneously (e.g. a piano chord). In this case if we offered "unity gain" via the plug-in, we'd clip the output of SCII onto the TDM bus.
With this in mind, we scaled down the output gain to avoid the problem (remember, we could only set a single level for the plug-in). If someone is running a single sample, it will be down 6dB, but there is 6dB of gain on a Pro Tools channel fader (and plenty more via a Time Adjuster plug-in) if needed. Other 'samplers' have this issue with digital output headroom, and some have adjustable headroom controls to take care of it . SampleCell uses the input fader in Pro Tools.
Another thing to keep in mind is that when a user scales a mix, it is probably not good practice to make the master meter hit "0" with a single sample from SCII. This would mean that the faders will need to be scaled down at some point anyway when attempting to balance elements relatively in the mix. Without this, you'll run out of "fader headroom" to move items like lead vocals to the forefront of the mix. When scaled back, the sweet spot ("0") of the fader for SCII tends to be the "right place" for most material to start with in a real mix.