Nobody cares about you not placing bets with the bookies either, you made a knowitall comment about something like you always do - perhaps you should have refrained. Furthermore, your comment about the outcome of any given season is also incredibly irrelevant to the discussion (and wuups, it paints Spurs in a better picture like it always does). Odds and percentages are put out before the season has started, which this discussion is about, and unless you can predict the future your arguments are worthless. Leicester won the league at 5000/1 two years ago so who the hell cares? The odds paints an "accurate" description of the strengths of every team on beforehand, whatever those variables might be - it's not an indication of the most popular clubs with fans betting the most amount of money (although correlation is there for sure).
The reason why Spurs are a bit lower is because they have probably hit some kind of glass ceiling (people's/bookies perception), if they could not beat Leicester or Chelsea they cannot win next season either. High top level, but not an extreme one. Even with an embarrassing Champions League-campaign and an even more traumatic Europa League-stint, they still could not manage to properly join the race for the title in their best ever Premier League-season. Next season removes some advantages and evens the playing field for all the top clubs minus Arsenal. All are in Europe, five of the six best teams are in the Champions League - no excuses.
I am however not saying United are a better team than Spurs at the moment, even though the odds might be lower. The odds would suggest that United have the ability to hit that extreme level if some variables come into place, most notably with Mourinho in charge, history, transfers (a game-changing player could sign for United next week, he would not go to Spurs) and so forth. Not to mention a few potentially great players that could hit form together like they did for Spurs did last year, most notably Mkhitaryan, Martial and Shaw. I am also not saying United have a bigger chance at winning compared to Spurs since they won the league a few years ago with Fergie/Van Persie - but winning is a habit and the organization is used to it. Mourinho knows how to win the league wherever he goes, Pochettino has never won a single thing in eight years - these things matter when odds are presented, which is what the discussion is about.