Your starting cash balance is made up of the matched/reconciled balances of the accounts you have included in Float.
The difference between a cash balance and a bank balance is this matching/reconciliation step.
Whereas the bank balance from Xero/QBO will reflect what is currently in the bank, the starting balance in Float will only take into account reconciled/matched transactions.
If there are unmatched/unreconciled transactions these won’t be included in the ‘Balance in Xero’/’In QuickBooks’ balances and therefore won’t show in Float.
When these transactions are reconciled/matched your starting cash balance will change and the transactions will pull through to Float.
It is therefore very important to reconcile/match your transactions in your accounting software as often as you can in order to maintain an accurate starting balance for your cash flow forecast in Float.