The chart was caiculated up to the 8 December 1979 and missed out a lot of big selling songs from December and Christmas. A few errors Sad Cafe, BRMB who compiled the chart wrote the sales figures the wrong way around so gave them a higher place than they should have had, also Ian Drury sales were under counted and he should have been in the Top 10.
So take the chart with a huge pinch of sale and enjoy it for the music rather than its accuracy.