Totally normal. All kids develop at a different pace, same with twins.

Having 4 single births then having twins, have found children develop at different rates. My twins are boy/girl and so expected some differences. There was not much in how they differed except their speech. My sis in law has triplets, her girls varied in different ways they progressed but her dr said it was normal.

Definitely. You also need to take into account if they were born earlier than 40 weeks as this make them younger again and likely to achieve milestone at different rates.

Absolutely! They are different people! You will especially find differences if one was born significantly smaller than the other, and started off “behind”. SOmetimes twins will reach usual milestones a little later than singletons but even that depends. As always get things checked if you are worried.