The MERs are lower simply because they are only available online meaning there is less overhead cost for TD to pass on. No need to over-complicate matters with speculations.I'm not sure what you are referring to as the couch potato portfolio. If you look on the TD website under mutual funds, you can look up the codes for the various funds.
TD Canadian Bond Index I series - TDB966
TD Canadian Bond Index e series - TDB909
Archanfel is right. The difference between these two funds (other than the fact that other mutual funds dealers may not sell the e-series fund) is the MER. The I-series version of the fund has an MER of 0.79%, whereas the e-series of the fund has an MER of 0.48%. This may relate to the the notion that other dealers and brokers won't sell e-series funds. Many funds have a commission/trailer fee structure. I think the MER is supposed to capture the effect of these fees. TD may not offer dealers/brokers commissions or trailers to sell the e-series, which explains why no other dealers/brokers would want to sell them, and why TD can offer a lower MER.