Ok think of the avg career ratings like this.
It's an OVERALL rating.
The opposition bowling strength IS considered. It is measured by the ratings of opposition bowlers.
The runs scored IS considered.
But, also, whether he made those runs in conditions where most of his peers failed to score decently IS considered and the ratings WERE adjusted accordingly.
And other factors too. A LOT more. All those play a role in the ratings.
Maybe when Javed Miandad played against WI, Pakistan as a whole also had a low scoring match. May be all his compatriot's scored even worse. Or may be the opposite happened. May be the pitches were all bowler friendly and produced a low scoring match. Maybe sometimes Javed failed while others did good. ALL these stats ARE taken into account in the rankings.