Just to clear up the "US" thing, there is nothing "US" about Sportsbook (yet). It is based on the
UK Fractional Wagering system. Fractional Odds are a UK thing. The US does whats called "Money Line" which is not available in Sportsbook yet (but will be someday).
The Decimals are just DISPLAY TOOLS to assist people in figuring out payouts that don't understand the Fractional Odds system. Below is something I wrote in another post explaining the decimal display values..
The two types of Decimals that follow the fractional odds are DISPLAY ONLY and are there to assist people in figuring out payouts. They have no baring on actual functionality. When you choose US, all you are doing is having a fraction converted to a decimal (as that is how the US converts fraction to decimal conversion). That's it, nothing more, nothing less. It doesn't change any functionality as the functionality is based on the Fractional Odds entered into the system. When you select European Decimal, all it does is add 1 to the value after the conversion from fraction to decimal. It still has no functionality as the functionality is still driven by the odds values you entered into the system. those fractional values are NOT converted, they are the same regardless of which DISPLAY you choose. It is simply a display that decimal odds betting people are used to seeing/understanding that they mentally can understand the payout. The actual wagering still works the same in both systems. When you place a wager in the decimal system (like you are used to) your stake is returned, but you are used to it being part of the return instead of two separate things. You receive the same amount of money regardless, you are just used to different "terms" and displays is all. I couldn't tell you how many times over the past decade I've had this same conversation and it always comes down to people using the decimal system not understanding how the fractional system works. vBookie with vBulletin is the same way. Its written using the UK Fractional Odds system and unlike sportsbook, it does not have a conversion for European decimal odds. It is purely UK Fractional odds based.
Examples:
When you create an outcome and enter the odds 50 to 100 and have the decimal display to US, the display looks like this: 50/100 (0.50).
When you create an outcome and enter the odds 50 to 100 and have the decimal display to EUROPEAN, the display looks like this: 50/100 (1.50) (simply adds 1 to the decimal to simulate Decimal Odds payout conversion).
As you can see, they are both still fractional odds based (50 to 100) which means that if you placed a $10 stake and that outcome was a winner, it would be processed like this. ($10*50)/100 = $5 Winnings + $10 Stake = $15 return.
In the fractional odds system, I start out with $10 in my pocket. I place that $10 stake on the outcome, the outcome wins, I get my $10 back, plus my $5 wining, so now I have $15 in my pocket. Total return is $15. Total Earnings is $5.
In the decimal odds system that you are used to, you start out with $10 in your pocket, you place a $10 wager on the outcome, the outcome wins and you win $15, so now you have $15 in your pocket. Total return is $15. Total Earnings is $5.
As you can see, both end up returning the same amount and earning the same amount. The difference is, one system separates the stakes from the winnings as they are two separate things in reality (for accounting). On electronic systems, they account for both the STAKE and the WINNINGS separately (and this includes decimal based systems).
Anywhere within Sportsbook that displays "Winnings", only actual "Winnings" are displayed, not Stake and Winnings. This gives a accurate picture of what people actually WON (Earned) vs what they already had to begin with plus winnings.
Hope that clears it up a little more.