Well, let's look at the exact text from the SRD.
SRD said:
Animal Companion (Ex): A druid may begin play with an animal companion selected from the following list: badger, camel, dire rat, dog, riding dog, eagle, hawk, horse (light or heavy), owl, pony, snake (Small or Medium viper), or wolf. If the campaign takes place wholly or partly in an aquatic environment, the following creatures are also available: crocodile, porpoise, Medium shark, and squid. This animal is a loyal companion that accompanies the druid on her adventures as appropriate for its kind.
A 1st-level druid’s companion is completely typical for its kind except as noted below. As a druid advances in level, the animal’s power increases as shown on the table. If a druid releases her companion from service, she may gain a new one by performing a ceremony requiring 24 uninterrupted hours of prayer. This ceremony can also replace an animal companion that has perished.
A druid of 4th level or higher may select from alternative lists of animals (see below). Should she select an animal companion from one of these alternative lists, the creature gains abilities as if the character’s druid level were lower than it actually is. Subtract the value indicated in the appropriate list header from the character’s druid level and compare the result with the druid level entry on the table to determine the animal companion’s powers. (If this adjustment would reduce the druid’s effective level to 0 or lower, she can’t have that animal as a companion.)
Okay, that's the main text.
SRD said:
THE DRUID’S ANIMAL COMPANION
A druid’s animal companion is different from a normal animal of its kind in many ways. The companion is treated as a magical beast, not an animal, for the purpose of all effects that depend on its type (though it retains an animal’s HD, base attack bonus, saves, skill points, and feats). It is superior to a normal animal of its kind and has special powers, as described
below.
Class Level Bonus HD Natural Armor Adj. Str/Dex Adj. Bonus Tricks Special
1st–2nd +0 +0 +0 1 Link, share spells
3rd–5th +2 +2 +1 2 Evasion
6th–8th +4 +4 +2 3 Devotion
9th–11th +6 +6 +3 4 Multiattack
12th–14th +8 +8 +4 5
15th–17th +10 +10 +5 6 Improved evasion
18th–20th +12 +12 +6 7
Animal Companion Basics: Use the base statistics for a creature of the companion’s kind, but make the following changes.
Class Level: The character’s druid level. The druid’s class levels stack with levels of any other classes that are entitled to an animal companion for the purpose of determining the companion’s abilities and the alternative lists available to the character.
In the text I highlighted, it's defining "Class Level" for purposes of using the chart, and says that it's the Druid's class level. It also says that the druid's class levels stack with the levels of any other classes... blah blah blah.
Now as far as I know, this is the only part of the rules that addresses combining levels of multiple classes that allow an Animal Companion.
And, like it or not, it doesn't say that we should stack "effective" or "caster" levels. It says stack the levels of the classes. Period.
So, despite Arrowhawk's repetitive redundancy, my reading comprehension is just fine. I'm reading the words written, exactly as written. I'm not trying to insert words that aren't there (unlike some), nor am I ignoring any words that are. I'm not trying to redefine any of the words (unlike some), nor using obscure meanings for any of them.
"levels of classes" means exactly that. No spin, no obscure or irrelevant references, just the rules as written, exactly as written.
If there's another section of the rules (SRD or hard copy) that tells us how to combine multiple classes w/regard to Animal Companions, somebody point to it. That would constitute a change in the ink on paper.
Until then, you're arguing with a book. Your arguments aren't going to change what the book says, no matter how persuasive you might be. The ink on the paper won't change.