Hi Elissa,
The problem partly relates to broad match data vs. exact match data for keyword traffic... Get those mixed up and it could be a massive difference from a handful to a thousands... and also keep in mind your ranking position... Obviously first place gets the most and it diminishes as you go down and beyond page 1 its real small.... The percentage breakdown is niche dependent - but I would say first position should get around ~30-40%, 2nd ~20%, 3rd ~15% .... and as you can see below 3rd position it just gets smaller and smaller.... So ranking 6th may not bring in that much traffic when you consider "exact match" data...
According to search engine stats and market share of G vs. Bing vs. Yahoo etc - its different with country to country... For most countries G is on top - easily with an 80% margin... China, Russia being notable exceptions.... For accurate stats - you can easily find them online...
Regarding your focus - indeed G being the biggest search engine by far is where you need to keep your eye on... Also consider Youtube as that is the second largest search engine with getting into video marketing....
Best of Luck,
Sid
