Why such low coverage on link units?

Ad units are doing fine.

[gmb21]

My ad units get over 99% coverage, but my link units are getting less than 10% coverage. Can anyone explain?

Thanks.

[koan]

10% of visitors clicked on a keyword in adlinks to view the ads and register the coverage, which is actually a lot.

[gmb21]

Thank you, Koan. So it means something completely different to Ad unit coverage which I understand to be whether or not there was ad inventory available to meet all the ad requests sent from your ad units. Strange that they call it the same thing!