2.25 really means 2 and 2.5, where half your wager amount will be on 2 and the other half will be on 2.5.
2.75 means 2.5 and 3, where half your wager amount will be on 2.5 and the other half will be on 3.
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As an example, suppose you bet $100 on under 2.25, so $50 will be on under 2 and the other $50 on the under 2.5. Let's say the final score is 1-1, which means you'll push on the under 2 and win on the under 2.5.
