Available balance and Funds Available Now diffrent

Hello,

Just wondering if someone knows the answer have asked amazon but just got the usal copy past answer.
so we disbursed today x amount much smaller than what would have been disbursed on the 7th . now we have gone on it says the Available balance is xxxx and Funds Available Now 0 and account level reserve is £5.00

my question is why are the available ballance and Funds Available Now diffrent isnt it the same thing? is it because we have disbured today thats why Funds Available Now is 0? rather than the available?