Just met another arti without fairly important scrolls (we won't go into which scrolls and why exactly and in which situation but anyway...). They had a UMD of 26 and the particular scroll that it would have been SERIOUSLY USEFUL for them to have used at that point of time has a UMD 36.
Now in my confusion I do remember that for a long time I remember thinking while learning this game "oh it's not 100%". It took a while for the penny to drop that that doesn't matter. That a 50% chance is 1 in 2 chances for the scroll to activate. A 75% chance is 3 in 4 chances, 12% is 1/8, and even 10% being 1 in 10 is still worth a shot and mostly likely worth it if you have a stack of 100 of the particular scroll.
The wiki says it semi clearly: "Scrolls can have less than a 100% chance of working even in normal use. If the character also has UMD trained, the better chance of success is used. " I'm one of those people who likes practical examples as per the paragraph above.