Bear with me...
But, let say child A turns 2 in June, and it's sunny and summer and long days. It's perfect weather for ... learning to ride a scooter. child As is out a lot of the days, and has the opportunity to practice. So are whizzing along by the end of June.
Child B turns 2 in November, and its cold, windy and rainy. Not so many opportunities perhaps this learn to ride their scooter. So end up being proficient at 2.5+ in the summer.
Replace scooting with any skill eg potty training, skiing etc
Yes. All children are different yadda yadda yadda. Yes. They have just as much opportunity because you could just take them in the rain (fun).
Anyway, just some light hearted musings.