The bookies ere offering 2 to 1 for the UK to leave.
And 1 to 2 for the UK to stay in.
Now the opinion polls are putting the two sides about the same, leave is about 2% ahead at present.
How the bookies work out the odds is designed to make sure they win and at worst don't loose money.
So why are the odds so different from the polls... Simple more money is being bet on stay so the odds on stay go down and the odds on leave go up.
Simple as that, the real odds are about 50/50 so a bet on leave is a worthwhile punt.
A bet on stay, regardless of your opinion or intention to vote is unwise.