its so you don't wake people up on your way to bed, wood stairs to start off cos there further away and still retaining the wooden floor look then as you get closer to the bedrooms you have carpets for quieter foot steps. totally makes sense don't ya think?
personally i like the, one side of the stairs carpetted and the other side not, that way you have a choice when climbing ones stairs, if you have your shoes on you would choose the wood floor option to escalate and de escalate, but if you only had socks or bare feet you would obviously choose the carpetted option. then if you were wearing slippers you could ascend and descend in the mddle but obviously you would have to choose the middle on your return to avoid uneven wear to the said slipper.