Just noticed this and asked support about it, they confirm it. Never saw anything regarding this before...
When you have your account set to decimal odds at the bookmaker family (bookmaker.eu, DSI...), they round the odds slightly to the lower side because they don't use the third decimal digit. This can affect your payout rate by over 1% in a lot of cases.
See this:
https://gyazo.com/a9a452fe6b9f327ec7a73b29b71a65c9
And then this:
https://gyazo.com/6ca89801899907bbc98502a61a347606
Same line, higher payout with the American odds.
Just letting everybody know you can get a slightly higher edge by setting your account to the American odds. 1% can make a big difference.