Even though the hospital has those rules in place, I would err on the side of caution when filling out AMCAS and secondary apps by listing only the number of hours actually volunteered.
I think it's the honest thing to do, too. The hospital probably does that because they expect their volunteers to volunteer a certain amount of time per month, and giving you extra hours for spending time their on the weekend is an incentive to ensure people do volunteer then (instead of play outside, yay!).
So, be honest and report the amount of time that you volunteered, not 1.5X those hours.
Honestly, it won't matter either way. More important than that is the total length of your activity (is this something you did for 6 months or 2 years?) and your ability to speak and write intelligently about your experiences in essays and interviews.
The only time I see hours being paramount to report is in physician shadowing.