The thing to remember through, is that the low rate of goalscoring in the early parts of the 20th century, especially into the early 1920's, was because the offside law was different. Up until 1925 the number of opponents required to be between the player and the opponent's goal line, was three.
The law has been this way since 1867 and by the early 1920's full-backs had developed an almost fool-proof tactic to catch attacking players offside. They would position themselves almost up to the halfway line, with one staggered slightly behind the other.
By keeping in a diagonal line, the full-backs could easily catch attacking players offside as they advanced past the first of the two. And should an attacking player run through after a long ball, he still had the other defending player to beat - and the goalkeeper. Defenders became so successful in using this strategy to catch attacking players offside that the game became extremely monotonous, with constant stoppages for offside offences.
So in 1925, in an effort to increase the number of goals being scored, the number of opponents was reduced from three to two.
The change clearly had the desired effect as the number of goals scored in the 1925/26 season was up by nearly 33% from the previous season.
So you shouldn't really include the years before 1925 if you're doing a comparison of goals per game, because the conditions were so significantly different.
