I think the fact that there are fewer seats does cover the lack of center bin. And as to why there's no center bin... In the case of a 747, I'd guess that the cabin feels more spacious without it. On an A340, the number of seats in the center block may be low enough, that these pax can just share the bins on either side.
If the seats have footrests, the life jackets are probably beneath the center armrest, instead of beneath the individual seats. In that case, there may be more underseat space. Just a guess