To start, the key thing here is Caster level. This is a specific function of a character's levels in a specific spell casting class. Page 208 of the Core Rules states, under "Caster Level":
A spell's power often depends on its caster level, which for most spellcasting characters is equal to her class level in the class she's using to cast the spell.
(This sentence actually removed the need to describe the idea of Caster Levels in specific spell descriptions, hence why Animate Dead doesn't mention it.)
Without special rules, these don't combine to anything. Your Cleric levels are cleric, bard are bard, Druid are druid. For this very reason, tables that allow Bard/Wizards to cast Animate Dead using both levels is absolutely not Rules as Written or Intended, regardless of the fact a bard can't normally cast Animate Dead. When casting Fireball, a Wizard 6/Bard 3 does not do 9d6 damage, they do 6d6 damage.
What that means to this original question is this: The class the animated dead are 'assigned' to depends specifically on which class is being used to cast the spell or the one used to create the scroll.
Animate Dead (Core Rules pg 241) says;
you can control only 4 HD worth of undead creatures per caster level.
It also says:
you can't create more HD of undead than twice your caster level with a single casting of animate dead.
This is important.
Next, we have the factor of the spell scroll. Page 490-491 of the Core Rules has this to say about spell scrolls:
Assume the scroll spell's caster level is always the minimum level required to cast the spell for the character who scribed the scroll, unless the scriber specifically desired otherwise.
This essentially means that this spell has already been cast, it's just waiting for you to unlock it at a later date. Your personal caster level isn't terribly relevant as far as the scroll is concerned.
So, your Animate Dead scroll, if prepared by a Cleric has to be at least caster level 5, and if Wizard, at least caster level 7. Most DMs, unless given special reasons to do so, generally figure loot scrolls to be minimum level. Those created during game are whatever caster level the scriber chose at the time, paying the appropriate costs to do so.
Putting these together we get the following results:
If the scroll is a Divine spell, it's on the Cleric spell list, so it can be cast without any difficulty. It creates undead based on the size of the onyx used in scribing the scroll, up to twice it's CL in Hit Dice. Because it's a costly item, the GM would have to know this, as you have to still use costly material components regardless of the scriber's feats. These are under the control of the caster, at 4HD per caster level of the SCROLL, not the character.
You shouldn't be using your Wizard caster levels here to fake the casting because, as a Divine scroll, you can already do that, and your Wizard levels have nothing to do with the CL (Caster Level) of the scroll anyhow.
If the scroll is Arcane, the same rules apply. Again, the caster level of the Wizard using the scroll is mostly irrelevant.
If a Bard or Rogue cast the spell using Use magic Device, the effects would be exactly the same.
Using a spell in memory, the rules are still workable by the same methods, only now you actually are using your own caster levels to determine effects, including how many undead you can have following you around at once.
In the case of that Cleric 5/Wizard 7, they can have up to 48 HD of undead. 20 as Cleric, 28 as Wizard.
Note: As spells have to complete, it's generally considered bad form to try and use non casting levels to grab that last few HD. If your Cleric 5 can only animate 12HD of undead, but they already have 10HD with them, they can't just use some of the Wizard pool to soak up the excess. That would require an additional Arcane casting of the spell, and not likely to grab hold of the already animated and wandering loose zombie.