@UKtoSK that's interesting. That may very well explain it, thank you.
@SRS1311 I'm glad I'm not the only one! At least she seems to enjoy the little I do for now.
Charlotte had some weight gain issues and so dropped from 91st centile to 9th but she is quite long. She has outgrown about half her 3-6 dresses, and her vests and sleep suits aren't far off either, depending on where they are from. She has little legs so her leggings are fine and will last for ages I think. I'm currently washing a good chunk of her 6-9 however as I think I will start moving her across soon, especially when some of her 6-9 vests are the same size as the 3-6 one from M&S. She is 23 weeks/5 months and a week.
My 3yo sounds very similar to your LO though. He stayed on the 91st and 95th for length and weight. He outgrew most of his clothes every 8 weeks for the first 8 months, I think, again depending on where they were from. He's in 3-4 bottoms and 4-5 tops and I expect he'll be in 5-6 tops before his birthday next summer.
Next and M&S seem to size bigger, and maybe TU. George, Mothercare and Nutmeg can be shorter.