The "IPv6" Capable column in the Comcast approved devices list (»mydeviceinfo.comcast.net/
) just indicates that Comcast has certified that it is IPv6 ready themselves and is compatible with their IPv6-enabled CMTSs.
All DOCSIS 3.0 devices must provide support for IPv6 as part of that specification.
It's likely that Comcast just hasn't got around to certifying the SB6141 model for IPv6 operation yet.
As the SB6120 and SB6121 models are shown as IPv6 capable in Comcast's list, I'm sure the SB6141 will someday too.