The game seems to randomize the span for each age. I haven't noticed it being due to happiness levels, but I haven't really picked it apart that much. Since they can be aged up with a cake at any point in each lifespan I generally just use the cake to age up the lag-behind. That won't work in the apocalypse, of course, but then I have never gotten very far in that. I do however generally have a house full of kids any time I play.
I haven't had the discrepancy with toddler twins but I have seen it with young adult twins and once or twice with teen twins.