PTSB Open24 banking & pending transactions

I noticed an odd one today - my online available balance was c. €18 less than the nominal balance around noon. Later this evening I checked again and the nominal and available balances were the same and the c. €18 transaction was listed. It was a Laser point of sale transaction. I guess that when I first looked the transaction was not fully reflected but the amount paid had been "reserved". Is it normal to see such transactions partially reflected as above?

Had the same issue with an AIB account (well, the GF did), ~€60 purchase of a train ticket by laser that we'd both forgotten about.

She queried it as she was phoning the 24hr banking on another matter and was told that the "balance" was the current balance as it stands, including any non processed transactions (not currently showing up in the transactions section). They commented that it was standard and happened on all accounts.

The only other time that I'd seen the "balance" and "available balance" differing was when chequest had been lodged that had not yet cleared. I didn't expect that you could have something similar for credits such as Laser transactions not yet posted but which had "reserved" the money!? No big deal but I was just curious about the mechanics/details of how this worked...

I think I enquired on this some years back with possibly Visa ac with AIB. The answer I got was it was in the system and I think they might have used the expression "shaddowed" for the transaction. In other words the balance is showing up correctly but the transaction itself if just not appearing yet.

Thanks. I never noticed it before. I can come up with all sorts of logical IT based explanations but it still surprised me from a purely administrative point of view.

A combination of the fact that the laser card gets used so much on my account and that PTSB are quite slow to process laser transactions and cheque lodgements means I can't ever remember seeing parity between "Account Balance" and "Available Balance".
I wonder which one they use for calculating interest.