There is no real chemical precision in the recipes, this isn't a mistake.
yes quantities are not exact (game balance vs reality) recipes do follow the same chemistry as reality , so saying outright its not a mistake vs a oversite for one small recipe that most wont use outside of stalling there factory is a bit presumptive.
Recipe quantities in the game are balanced for gameplay rather than following exact chemistry, so the electrolyzer's hydrogen to oxygen ratio is not treated as a bug.