Lay Betting Must read

i am a begginer and before i bet all my money away i just want someone to reassure me that my understanding of lay betting is correct.

so if i chose the market total score. and layed all. and put £2 stake. my total liability will be £418 if 3-3 is the score.

but as i understand. i get payed out on all the other scores. which would add up to about £1200.

this seems to good to be true. why doesnt everyone do that. surely ive got something wrong here