Why odds formats do not change the bet
Decimal, fractional and American are three spellings of one number. Switching format changes what you read, not what you are offered.
A price of 2.50 is a price of 3/2 is a price of +150. All three describe
the same offer: risk one unit, and if it lands you hold two and a half. The
choice between them is a choice about which one you read without stopping to
think.
The one place presentation can mislead
The formats themselves do not round: 28/25 and 2.12 are exactly the same.
A platform can choose a coarser display, but a displayed 11/10 means 2.10;
there is no hidden 2.12 inside it. When comparing books, use the stated
return or a decimal conversion with enough precision. The displayed price,
not the notation used for it, determines the contractual return.
American odds have a discontinuity at even money that trips people up. +100
and -100 are the same price, and there is nothing between them. A move from
+105 to -105 is a small move that looks like it crossed something
important.
Why we let you choose
A reader in Lyon and a reader in Ohio are looking at the same board and reading two different alphabets. Forcing either into the other's notation adds a translation step to every glance, and a translation step is where mistakes live. So the format is a setting, defaulting to whatever is normal where you are, and every figure in the product follows it.
None of this touches the arithmetic. Whatever notation is on screen, the thing being compared is the gap between our number and the price, and that gap is measured in probability.
What this does not tell you
Format has no bearing on whether a price is good. A beautifully readable
+150 is a return of 2.50, so it is worse than an awkward 13/8, which is
2.625, for the same outcome. Readability is a convenience, not a signal.
Common questions
Is 2.50 the same as +150?
Yes. Both imply a 40% chance and both return 2.5 times the stake including it. They are the same offer written for different readers.
Which odds format is best?
The one you read fastest. Decimal makes the arithmetic easiest, fractional is conventional in British racing, and American is standard in the United States.
Do some formats hide a worse price?
No format changes an exactly converted price. A platform can round a displayed value in any notation, so compare the stated return or a sufficiently precise decimal price.