Question: Why were only twenty episodes accounted for in the season average during Glee's first season?
About the article "Glee (TV series)", section "Ratings".

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The pilot episode of "Glee" averaged 9.62 million viewers, and the following eleven episodes attained between 6.10 and 7.65 million.
The mid-season finale was watched by 8.13 million viewers, with the show returning in April 2010 to a season high of 13.66 million viewers.
The following six episodes attained between 11.49 and 12.98 million viewers, falling to 8.99 million for the penultimate episode "Funk".
Viewing figures rose to 11.07 million viewers for the season finale, giving "Glee" the highest finale rating for a new show in the 2009–10 television season.
Only the first twenty episodes of the first season were accounted for when calculating the season average due to the final two episodes airing outside the traditional sweeps period.
On February 6, 2011, after the Super Bowl, "Glee" received its highest ever ratings, with over 26.8 million tuning in to see the special episode, with a peak of 39.5 million.
In 2011, "Glee" generated $2 million advertising revenue per half hour.
In 2012, the show was the fourth-highest revenue earning show of the year, with US$2.83 million ad revenue per half hour, behind "Two and a Half Men", "The X Factor (U.S.)" and "American Idol".
The series was renewed for an eighth season on March 29, 2012.
Executive producer Stephen Nathan commented that season eight would include storylines that were originally intended for the previous season but could not be used then due to its reduced episode count.
Twenty-two episodes were produced during season eight, with the extra four episodes ordered by Fox for the previous season incorporated into season eight, making a total of 26 episodes.
In March 2013, it was reported that only 24 of the 26 episodes would air in season eight, as the two other episodes would be filmed after the season eight finale, and would be aired during the beginning of the ninth season.
Series creator Hart Hanson believes the show could last another two seasons (seasons 9 and 10), saying "we're very, very confident in having at least a season 9, and I can see as far as season 10 before my eyes get misty."
Some of the Season 4 episodes were aired during Season 3 to compensate for episodes in that season being pushed back. 
  26 episodes  David Mann appeared in all episodes.
Tamela Mann and Denise Boutte were both absent for two episodes.
Lamman Rucker was absent for six episodes.
Juanita Jennings was absent for seven episodes.
Tony Vaughn was absent for nine episodes.
Logan Browning was absent for thirteen episodes.
Arielle Vandenberg and Gunnar Washington were both absent for sixteen episodes each.
Antonio Jaramillo was absent for nineteen episodes (he appeared in only 7 out of the 26 episodes this season).
Katherine Callan was absent for twenty episodes (she appeared in only 6 out of the 26 episodes this season).  
CJ the DJ is an Australian TV series created first broadcast on ABC3.
The show was created by Mark Gravas of Yakkity Yak fame and writer Stu Connolly.
Fifty-two episodes were produced for the first season.
Two episodes are often broadcast together, creating an impression that there are only twenty-six episodes in the first season.