If you add selections of a 1000/1 then you will get silly odds in a multi bet. The returns you are quoted are the maximum potential returns if every single one of your selections wins.
Ah, I see, I was under the impression from what I looked at online that a treble with 4 selections will return the price on the bet slip if 3 of the 4 are correct. Apologies
No it's maximum payout, it's the only figure they can quote at time of placing bet, not conceivable for them to inform you of every possible outcome. You can work it out yourself though.