Eldar psykers could potentially be extremely powerful but they have their hands back tied due to the relationship between Eldar and Slaanesh. A Farseer could make use of some awesome powers but if he chose to do so he would most likely be eternally damned due to being consumed by daemons.
More than anything that scale is focused on the amount of power a psyker can safely wield as its main criterion for level. As Benis and Sir-Godspeed pointed out, it is not safe for Eldar to actually use this amount of raw power, they would be almost certainly eaten. However, we see that they are fully capable
of doing so, they use abilities in battle that are comparable to those of the other races' most powerful combat psykers' typical skills. But with the limitations they must deal with they aren't seen wielding the power other races use without consideration. For example, Librarians and Warlocks (or Farseers) alike can unleash a torrent of warp energy that obliterates a bunch of armored warriors, this is clearly at the Beta or Alpha level. (Or Gamma, depending on your interpretation of the levels.) While the Librarian is perfectly willing to use this level of power to try and hurl an enemy into the warp directly, the Warlock or Farseer can't/won't try and open up holes to the warp willy-nilly. Eldar have the innate ability to use psychic powers at the alpha level with lots of practice, but typically can't actually do it, it's too dangerous.
However, even considering their 'true' ablilities, amongst the Eldar you won't normally see the super-high levels of power that you're talking about. (One psyker wielding enough power to combat a titan, etc.) Those are beyond rare, even creatures that exist for no other purpose than to channel warp energy, like tyranid zoanthropes, cannot normally use that level of power. Alpha+ psykers are the kind of individuals that are behind unexplained colony disappearances, cause sector-wide wars, or get the Grey Knights to dispatch multiple strike cruisers. I would think it would be as common for an Imperial inquisitor to encounter an alpha+ psyker as it would be for a normal imperial citizen to encounter an Eldar. That is to say, not at all. (And inquisitors encounter Eldar all the time.)
The skill and reliability with which the Eldar use warp energy is beyond compare though, and certainly at a higher level than any others, which, as has been pointed out already, contributes a lot
to how high they are on the psyker scale. They have established ways of doing things that are better than any others, and part of the reason almost any Eldar can, with enough training, reach alpha levels of mastery is because they just (although it's actually a lot harder than that) have to "follow the manual." There are protections and safeguards that just aren't available to other races, from the rune system, to the knowledge of how exactly all this psychic stuff works, to effective protective gear. This compared to Imperial psykers, who are basically throwing that power around with no training or protection at all.
Remember, the Eldar are a pretty stable race, they've existed in their current form for millions of years. Humans are just starting to develop psychic abilities now yet can still channel the same amounts of power as the Eldar or more
without any of the knowledge or skills the Eldar have. They certainly seem on course to surpass the Eldar in terms of psychic power if humanity doesn't die out within the next few hundred thousand years.
For one example of the limitations of Eldar power, consider the duel between Abaddon and Eldrad. Eldrad is widely considered one of the most powerful known Eldar psykers, and yet he had to actually fight
Abaddon, like with his fists. (Well, not literally, but fight him hand-to-hand.) If he was at the alpha+ level, even with his protections from the chaos gods, Eldrad could have just flattened Abaddon with his psychic might, or blasted the section of ship Abaddon was in into space. But not only was he unable to even affect Abaddon with psychic powers, he was barely able to protect himself.