Chart positions are deceptive. Maiden singles and albums notoriously sell well upon their release, which explains their high chart positions, but they always dropped significantly after the first few weeks. I remember reading an interview from 1988 in which Adrian, I think, said that Can I Play With Madness was the first single which he considered a big hit, because it did not peak and then drop, but made the top ten and stayed there for a significant period of time. A record that hits a peak and quickly drops arguably makes less of an impact on mainstream culture than one that doesn't go as high but stays in the chart for a prolonged period of time.
Meat Loaf's Bat Out of Hell, for instance, is one of the best selling albums of all time (43 million units), and virtually every western household that existed around the late seventies has the record in its collection, but it's highest chart positions in the UK and the US were 9 and 14, respectively.