Question about pre-authorisation at pump

I bought fuel at Asda pay at pump.
The message on the screen said pre-authorising for £1.00
Then it said please fuel up to £100.
I put in £39 of fuel.
The Curve app said my purchase was £50, so I sent a query to CS to ask why the random amount. They didn’t read my query I just got a standard message back about refunds but no explanation of why it was £50 (I hate it when that happens!)
It has since corrected, does anyone know was the £50 the pre-auth? If so is it always £50?
(If so the message on the pump was wrong)
Or is there some other explanation?

£50 was the pre-authorised amount. I always get that as the pre-auth amount, real amount filled up gets corrected in about two working days.
You can also have a look at this update from MasterCard about pay at Pump transactions

Also this Post in the curve community

Thanks for the explanation.
The message on the pump misled, and hopefully your reply will help someone else in the future.