This is my current Annual Interest Rate:

ANNUAL INTEREST RATE
4.39% (+0.31%)*
Seems like you haven't understood the system yet even though hosseinimr93 has explained it above, and id5000 has shown you the calculation. Freebitco team has mentioned this in the premium membership page
Receive 1% cashback on your MULTIPLY BTC / BETTING wagers, 16 free Wheel of Fortune spins daily and 25% increase to interest rates by buying and holding FUN tokens in your FreeBitco.in account!
They have said about the increase of the 4.08% interest rate by the 'X' percentage. The 7.5% increase of 4.08 is (7.5÷100) × 4.08 = 0.306%. It has been rounded up to 0.31%. Therefore, your interest boost amount is accurate there. You have to understand the system properly.