That certainly makes sense. Has Hillary been winning most/all of the states so far that had early voting? I've been paying attention to the race but honestly don't remember which states had early voting and which ones didn't. Many were caucuses so far.
So primaries that encourage huge voter turnout favors Sanders, but Hillary's dedication to getting the early vote adds security for her numbers?
Actually, most of the states Bernie has won have had fairly low turnout. He won Kansas, for example, but got fewer votes in the entire state than Hillary did in one single county in Florida.
Some of the bigger states with early voting were Ohio, Florida, Texas and North Carolina. Hillary's early vote lead in Florida was enough to make it nearly impossible for Bernie to have gotten within 10 points even if pretty much no one had voted for her on election day.
Early voters tend to be older, women and minorities. someone who votes early has a chance to fix any errors that ay come up. For example, some people in Arizona have to vote provisionally because they never changed their party registration. Had they been targeted early by Bernie's campaign for early voting, they could have potentially had this fixed early in the cycle.
Early voting also shows which campaign has the better ground game. Getting people to vote early, tracking them and their ballots is a far better use of time than hounding people and saying "You can vote on this day." It removes excuses, it makes the process more convenient, and it lets the campaign know where they need to allocate resources.
For example, say you know that you expect 10,000 votes from Polk County and 10,000 votes from Orange county. Based on your early voting targets, you know that 8,000 people in Polk have already voted, but only 2,000 in Orange have done so. You know to shift your candidate and resources to Orange. If you're just sitting there on election day saying "We need 10,000 let's get them there in the next 24 hours" it's a lot harder.