OK, let's kill this by looking at the question from another angle:
What does "Emulate a class feature" do?
Emulate a Class Feature: Sometimes you need to use a class feature to activate a magic item. In this case, your effective level in the emulated class equals your Use Magic Device check result minus 20. This skill does not let you actually use the class feature of another class. It just lets you activate items as if you had that class feature. If the class whose feature you are emulating has an alignment requirement, you must meet it, either honestly or by emulating an appropriate alignment with a separate Use Magic Device check (see above).
So, if the DC to activate a scroll is 20+CasterLevel, and the DC to have the Spells class feature of (Class that wrote the spell) at CasterLevel is also 20+CasterLevel....
I read the Read a Scroll text such that this is equivalent in meaning:
Use a Scroll: Normally, to cast a spell from a scroll, you must have the scroll's spell on your class spell list. Use Magic Device allows you to emulate the Spells class feature as above - remember that the DC is equal to 20 + the caster level of the spell you are trying to cast from the scroll. In addition, casting a spell from a scroll requires a minimum score (10 + spell level) in the appropriate ability. If you don't have a sufficient score in that ability, you must emulate the ability score with a separate Use Magic Device check.
That makes it one check for the class feature and one possibly skipped check for the ability score.
I certainly see no cause to interpret that as two checks at the same high DC for every activation.