Your available balance only shows the money you actually have in your account. It doesn’t include your Arranged Overdraft if you have one.
Sometimes you’ll see a difference between the balance and the transactions showing on your account and your available balance.
This happens when you’ve made card payments that have been authorised but these haven’t yet reached your account. The reason for the temporary difference is that these are deducted from your available balance at the time they are authorised.