So maybe you were just unlucky?
This was the Calmac summer timetable which ran at the same time/schecule every day, regardless of tide. The reason it could do this, is that "they" made a big investment in the linkspan at Mallaig, new piers on each of the Small Isles (so the ferry could physically dock at them all for the first time, rather than use a tender), and in a special new ferry, Loch Nevis which has a folding ramp so long it's effectively got it's own linkspan meaning it can dock at any of the piers at any tide.
So - why go to all the trouble of investing in a new ferry service which can keep a regular timetable, only to not synchronise it with connecting onwards public transport options!
This was back in 2005/6 and things may have improved now. Only the winter timetable is on the Calmac site, and this doesn't have the offending Saturday sailing from Canna on it.