Why the difference in delivery date RM24/Tracked 24?

So I post most of my stuff RM48 but I offer a Premium Delivery option for faster delivery when I send Tracked 24.

I use Amazon Buy Shipping.

Now when someone upgrades to Premium delivery, I am finding that I have to manually change the delivery option myself before pressing Buy Shipping.

Sometimes it will be defaulting to RM24 and others to RM48 (my standard service anyway). I cannot figure out why as its so random.

Anyway, today I was manually changing all the orders that had upgraded to Premium Delivery to Tracked 24 when I noticed something else odd. According to Amazon if I sent RM48 expected delivery date was in 2 days time. If I sent RM24 or 1st Class expected delivery date was tomorrow. However, if I selected Tracked 24 expected delivery date was again in 2 days time!!!

What - can anyone explain why Tracked 24 takes 2 days whereas RM24 takes 1 day?

If Tracked 24 takes as long for delivery as RM48 - why am I bothering to send Tracked 24 when anyone upgrades postage. I may as well just send RM24 and save the expense of Tracked.

Or is this just an another Amazon bug?