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"Should they be hugely different numbers? Not in any reasonable system."

The test of any reasonable system is a wholly unreasonable example- if the system is sound, then any example, no matter how absurd, will prove the systems merits. My three examples are going to show how "any reasonable system" would break down with regard to forming any sort of numeric or formulaic relationship between CR and level adjustment. I'll leave aside that most "reasonable" systems would be either too complex or too inelegant for use in most campaigns; that's an issue for another day.

Example 1: Noncombative Skill Bonuses
The Blorns are a humanoid offshoot that gain a +20 bonus to Perform and Diplomacy checks, meaning that they can usually charm anyone out of their money and goods, and almost always can avoid dangerous combat by creating proxies and intermediaries willing to fight on their behalf or simply unwilling to cause them harm.

For a PC, this bonus is tremendous; most encounters fall apart because the character can charm his way out of pretty much anything. Clearly, this is worth a hefty level adjustment, because it greatly affects the character's ability to survive through longterm play and to influence the members of the campaign world.

For an NPC, this bonus is worth nothing. Since PCs are entirely unaffected by the attitude-changing effects of, when it comes down to it, the ability of a Blorn to pose an individual threat to a party of four PCs is not influenced at all by this bonus. Since this is what CR measures, the Blorn would have a CR comparable to a human's.

Conclusion: Skills are one area where ECL and CR can be completely unrelated.

Example 2: Power of Survivability
The Foo is another humanoid offshoot, this one blessed with incredible healing powers. Once per hour, after one minute of complete rest, a Foo in affected as by a heal spell cast by a caster of his character level (minimum 11th level).

For a PC, this power is incredibly useful, and would force a massive shift in ECL (in the form of a large level adjustment) in order to account for the fact that this character can survive encounter after encounter, where most characters of similar level would last at most three encounters before needing a complete day's rest or more.

For an NPC, this power is again almost useless. Since CR is a measurement of a creature's threat to a party- in effect, a measure of its survivability over the course of a few rounds rather than 20 levels- and since no encounter is realistically designed to allow for a full minute's rest, this power is, at best, worth a minor bump in CR.

Conclusion: Healing powers and spell-like abilities in general are an area in which systems relating CR to ECL begin to break down.

Example 3: The HD-based Template
This is an example that already exists (in the form of the half-celestial and half-fiend, among others): some templates will have a straight +4 as a level adjustment, but different CR changes based on the number of HD the creature with the template has. This is because level adjustments are meant to measure the effect of a group of abilities over the course of advancement, while challenge rating shifts measure the additional threat those abilities represent at a given point in time. The template doesn't become more or less powerful; the creature that possesses the template gains additional benefit as it advances, changing the threat that creature poses based on the number of Hit Dice it possesses.

Conclusion: Templates and abilities which depend on Hit Dice force any formulaic relationship between CR and ECL to break down.

That's just three examples where any reasonable system would be forced to become more complex and less easy to use, or slowly begin to break down in its ability to cover everything. I could devise more, but the above are the sorts of things that led away from a direct CR/ECL relationship in the first place.