They had no scanners back then so they had to estimate the number of sold units. Because of that the charts where not very reliable, especially if you consider that the airplay the records got on radio stations could influence the chart position, too. And the third thing is, that the record dealers could choose, if they would put a certain record in the country-, pop- or whatever charts. So it could happen, that your record got a lot of airplay and everybody thinks it's a pop record and you'll have a high position in the charts. If a part of the record dealers put your record in the country charts and some think it's a pop record your record would not have charted high in any of the charts.
I wouldn't give too much of chart positions of records sold prior to the early 1990's.