Its usually the use of gambling terms or imagery that causes this.
"When Poker Meets Solitaire" with standard playing cards is the issue.
Australia recently passed legislation where
“in-game purchases with an element of chance,” get an M (recommended for but not restricted to 15 or older) and games that feature “simulated gambling” get R18+(restricted to 18 or older).
If this title was classified today it would have the same issue there.