I have this untested, wild theory that their brains focus on developing rapidly on certain things at a time.
... and I second that.

Have got 2 children myself and had to learn (especially with the first one), that nothing works like the textbooks say. Especially in regards to timing of "milestones".
Both my children 'only' made their first attempts to walk with 15 months and are now accomplished dancers, ....

As said previously, if everthing else movement-wise is normal, don't worry. And if you cannot stop worrying, see your GP.

Everyone in their own time ...
Regards,
Fyzzio